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From c5386144fbf09f628148101bc674e2421cdd16e3 Mon Sep 17 00:00:00 2001
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Message-Id: <c5386144fbf09f628148101bc674e2421cdd16e3.1387382496.git.minovotn@redhat.com>
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From: Nigel Croxon <ncroxon@redhat.com>
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Date: Thu, 14 Nov 2013 22:52:37 +0100
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Subject: [PATCH 01/46] add a header file for atomic operations
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RH-Author: Nigel Croxon <ncroxon@redhat.com>
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Message-id: <1384469598-13137-2-git-send-email-ncroxon@redhat.com>
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Patchwork-id: 55686
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O-Subject: [RHEL7.0 PATCH 01/42] add a header file for atomic operations
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Bugzilla: 1011720
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RH-Acked-by: Orit Wasserman <owasserm@redhat.com>
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RH-Acked-by: Amit Shah <amit.shah@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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Bugzilla: 1011720
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https://bugzilla.redhat.com/show_bug.cgi?id=1011720
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>From commit ID:
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commit 5444e768ee1abe6e021bece19a9a932351f88c88
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Author: Paolo Bonzini <pbonzini@redhat.com>
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Date:   Mon May 13 13:29:47 2013 +0200
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    add a header file for atomic operations
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    We're already using them in several places, but __sync builtins are just
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    too ugly to type, and do not provide seqcst load/store operations.
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    Reviewed-by: Richard Henderson <rth@twiddle.net>
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    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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---
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 docs/atomics.txt         |  352 ++++++++++++++++++++++++++++++++++++++++++++++
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 hw/display/qxl.c         |    3 +-
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 hw/virtio/vhost.c        |    9 +-
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 include/qemu/atomic.h    |  198 ++++++++++++++++++++++----
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 migration.c              |    3 +-
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 tests/test-thread-pool.c |    8 +-
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 6 files changed, 529 insertions(+), 44 deletions(-)
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 create mode 100644 docs/atomics.txt
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Signed-off-by: Michal Novotny <minovotn@redhat.com>
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---
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 docs/atomics.txt         | 352 +++++++++++++++++++++++++++++++++++++++++++++++
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 hw/display/qxl.c         |   3 +-
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 hw/virtio/vhost.c        |   9 +-
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 include/qemu/atomic.h    | 198 +++++++++++++++++++++-----
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 migration.c              |   3 +-
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 tests/test-thread-pool.c |   8 +-
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 6 files changed, 529 insertions(+), 44 deletions(-)
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 create mode 100644 docs/atomics.txt
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diff --git a/docs/atomics.txt b/docs/atomics.txt
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new file mode 100644
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index 0000000..6f2997b
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--- /dev/null
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+++ b/docs/atomics.txt
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@@ -0,0 +1,352 @@
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+CPUs perform independent memory operations effectively in random order.
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+but this can be a problem for CPU-CPU interaction (including interactions
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+between QEMU and the guest).  Multi-threaded programs use various tools
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+to instruct the compiler and the CPU to restrict the order to something
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+that is consistent with the expectations of the programmer.
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+
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+The most basic tool is locking.  Mutexes, condition variables and
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+semaphores are used in QEMU, and should be the default approach to
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+synchronization.  Anything else is considerably harder, but it's
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+also justified more often than one would like.  The two tools that
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+are provided by qemu/atomic.h are memory barriers and atomic operations.
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+
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+Macros defined by qemu/atomic.h fall in three camps:
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+
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+- compiler barriers: barrier();
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+
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+- weak atomic access and manual memory barriers: atomic_read(),
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+  atomic_set(), smp_rmb(), smp_wmb(), smp_mb(), smp_read_barrier_depends();
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+
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+- sequentially consistent atomic access: everything else.
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+
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+
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+COMPILER MEMORY BARRIER
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+=======================
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+
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+barrier() prevents the compiler from moving the memory accesses either
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+side of it to the other side.  The compiler barrier has no direct effect
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+on the CPU, which may then reorder things however it wishes.
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+
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+barrier() is mostly used within qemu/atomic.h itself.  On some
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+architectures, CPU guarantees are strong enough that blocking compiler
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+optimizations already ensures the correct order of execution.  In this
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+case, qemu/atomic.h will reduce stronger memory barriers to simple
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+compiler barriers.
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+
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+Still, barrier() can be useful when writing code that can be interrupted
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+by signal handlers.
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+
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+
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+SEQUENTIALLY CONSISTENT ATOMIC ACCESS
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+=====================================
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+
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+Most of the operations in the qemu/atomic.h header ensure *sequential
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+consistency*, where "the result of any execution is the same as if the
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+operations of all the processors were executed in some sequential order,
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+and the operations of each individual processor appear in this sequence
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+in the order specified by its program".
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+
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+qemu/atomic.h provides the following set of atomic read-modify-write
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+operations:
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+
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+    void atomic_inc(ptr)
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+    void atomic_dec(ptr)
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+    void atomic_add(ptr, val)
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+    void atomic_sub(ptr, val)
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+    void atomic_and(ptr, val)
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+    void atomic_or(ptr, val)
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+
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+    typeof(*ptr) atomic_fetch_inc(ptr)
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+    typeof(*ptr) atomic_fetch_dec(ptr)
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+    typeof(*ptr) atomic_fetch_add(ptr, val)
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+    typeof(*ptr) atomic_fetch_sub(ptr, val)
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+    typeof(*ptr) atomic_fetch_and(ptr, val)
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+    typeof(*ptr) atomic_fetch_or(ptr, val)
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+    typeof(*ptr) atomic_xchg(ptr, val
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+    typeof(*ptr) atomic_cmpxchg(ptr, old, new)
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+
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+all of which return the old value of *ptr.  These operations are
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+polymorphic; they operate on any type that is as wide as an int.
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+
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+Sequentially consistent loads and stores can be done using:
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+
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+    atomic_fetch_add(ptr, 0) for loads
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+    atomic_xchg(ptr, val) for stores
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+
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+However, they are quite expensive on some platforms, notably POWER and
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+ARM.  Therefore, qemu/atomic.h provides two primitives with slightly
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+weaker constraints:
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+
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+    typeof(*ptr) atomic_mb_read(ptr)
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+    void         atomic_mb_set(ptr, val)
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+
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+The semantics of these primitives map to Java volatile variables,
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+and are strongly related to memory barriers as used in the Linux
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+kernel (see below).
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+
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+As long as you use atomic_mb_read and atomic_mb_set, accesses cannot
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+be reordered with each other, and it is also not possible to reorder
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+"normal" accesses around them.
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+
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+However, and this is the important difference between
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+atomic_mb_read/atomic_mb_set and sequential consistency, it is important
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+for both threads to access the same volatile variable.  It is not the
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+case that everything visible to thread A when it writes volatile field f
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+becomes visible to thread B after it reads volatile field g. The store
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+and load have to "match" (i.e., be performed on the same volatile
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+field) to achieve the right semantics.
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+
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+
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+These operations operate on any type that is as wide as an int or smaller.
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+
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+
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+WEAK ATOMIC ACCESS AND MANUAL MEMORY BARRIERS
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+=============================================
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+
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+Compared to sequentially consistent atomic access, programming with
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+weaker consistency models can be considerably more complicated.
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+In general, if the algorithm you are writing includes both writes
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+and reads on the same side, it is generally simpler to use sequentially
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+consistent primitives.
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+
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+When using this model, variables are accessed with atomic_read() and
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+atomic_set(), and restrictions to the ordering of accesses is enforced
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+using the smp_rmb(), smp_wmb(), smp_mb() and smp_read_barrier_depends()
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+memory barriers.
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+
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+atomic_read() and atomic_set() prevents the compiler from using
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+optimizations that might otherwise optimize accesses out of existence
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+on the one hand, or that might create unsolicited accesses on the other.
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+In general this should not have any effect, because the same compiler
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+barriers are already implied by memory barriers.  However, it is useful
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+to do so, because it tells readers which variables are shared with
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+other threads, and which are local to the current thread or protected
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+by other, more mundane means.
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+
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+Memory barriers control the order of references to shared memory.
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+They come in four kinds:
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+
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+- smp_rmb() guarantees that all the LOAD operations specified before
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+  the barrier will appear to happen before all the LOAD operations
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+  specified after the barrier with respect to the other components of
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+  the system.
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+
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+  In other words, smp_rmb() puts a partial ordering on loads, but is not
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+  required to have any effect on stores.
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+
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+- smp_wmb() guarantees that all the STORE operations specified before
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+  the barrier will appear to happen before all the STORE operations
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+  specified after the barrier with respect to the other components of
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+  the system.
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+
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+  In other words, smp_wmb() puts a partial ordering on stores, but is not
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+  required to have any effect on loads.
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+
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+- smp_mb() guarantees that all the LOAD and STORE operations specified
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+  before the barrier will appear to happen before all the LOAD and
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+  STORE operations specified after the barrier with respect to the other
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+  components of the system.
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+
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+  smp_mb() puts a partial ordering on both loads and stores.  It is
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+  stronger than both a read and a write memory barrier; it implies both
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+  smp_rmb() and smp_wmb(), but it also prevents STOREs coming before the
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+  barrier from overtaking LOADs coming after the barrier and vice versa.
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+
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+- smp_read_barrier_depends() is a weaker kind of read barrier.  On
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+  most processors, whenever two loads are performed such that the
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+  second depends on the result of the first (e.g., the first load
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+  retrieves the address to which the second load will be directed),
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+  the processor will guarantee that the first LOAD will appear to happen
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+  before the second with respect to the other components of the system.
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+  However, this is not always true---for example, it was not true on
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+  Alpha processors.  Whenever this kind of access happens to shared
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+  memory (that is not protected by a lock), a read barrier is needed,
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+  and smp_read_barrier_depends() can be used instead of smp_rmb().
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+
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+  Note that the first load really has to have a _data_ dependency and not
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+  a control dependency.  If the address for the second load is dependent
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+  on the first load, but the dependency is through a conditional rather
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+  than actually loading the address itself, then it's a _control_
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+  dependency and a full read barrier or better is required.
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+
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+
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+This is the set of barriers that is required *between* two atomic_read()
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+and atomic_set() operations to achieve sequential consistency:
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+
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+                    |               2nd operation             |
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+                    |-----------------------------------------|
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+     1st operation  | (after last) | atomic_read | atomic_set |
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+     ---------------+--------------+-------------+------------|
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+     (before first) |              | none        | smp_wmb()  |
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+     ---------------+--------------+-------------+------------|
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+     atomic_read    | smp_rmb()    | smp_rmb()*  | **         |
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+     ---------------+--------------+-------------+------------|
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+     atomic_set     | none         | smp_mb()*** | smp_wmb()  |
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+     ---------------+--------------+-------------+------------|
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+
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+       * Or smp_read_barrier_depends().
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+
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+      ** This requires a load-store barrier.  How to achieve this varies
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+         depending on the machine, but in practice smp_rmb()+smp_wmb()
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+         should have the desired effect.  For example, on PowerPC the
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+         lwsync instruction is a combined load-load, load-store and
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+         store-store barrier.
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+
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+     *** This requires a store-load barrier.  On most machines, the only
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+         way to achieve this is a full barrier.
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+
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+
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+You can see that the two possible definitions of atomic_mb_read()
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+and atomic_mb_set() are the following:
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+
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+    1) atomic_mb_read(p)   = atomic_read(p); smp_rmb()
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+       atomic_mb_set(p, v) = smp_wmb(); atomic_set(p, v); smp_mb()
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+
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+    2) atomic_mb_read(p)   = smp_mb() atomic_read(p); smp_rmb()
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+       atomic_mb_set(p, v) = smp_wmb(); atomic_set(p, v);
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+
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+Usually the former is used, because smp_mb() is expensive and a program
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+normally has more reads than writes.  Therefore it makes more sense to
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+make atomic_mb_set() the more expensive operation.
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+
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+There are two common cases in which atomic_mb_read and atomic_mb_set
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+generate too many memory barriers, and thus it can be useful to manually
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+place barriers instead:
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+
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+- when a data structure has one thread that is always a writer
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+  and one thread that is always a reader, manual placement of
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+  memory barriers makes the write side faster.  Furthermore,
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+  correctness is easy to check for in this case using the "pairing"
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+  trick that is explained below:
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+
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+     thread 1                                thread 1
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+     -------------------------               ------------------------
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+     (other writes)
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+                                             smp_wmb()
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+     atomic_mb_set(&a, x)                    atomic_set(&a, x)
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+                                             smp_wmb()
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+     atomic_mb_set(&b, y)                    atomic_set(&b, y)
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+
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+                                       =>
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+     thread 2                                thread 2
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+     -------------------------               ------------------------
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+     y = atomic_mb_read(&b)                  y = atomic_read(&b)
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+                                             smp_rmb()
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+     x = atomic_mb_read(&a)                  x = atomic_read(&a)
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+                                             smp_rmb()
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+
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+- sometimes, a thread is accessing many variables that are otherwise
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+  unrelated to each other (for example because, apart from the current
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+  thread, exactly one other thread will read or write each of these
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+  variables).  In this case, it is possible to "hoist" the implicit
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+  barriers provided by atomic_mb_read() and atomic_mb_set() outside
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+  a loop.  For example, the above definition atomic_mb_read() gives
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+  the following transformation:
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+
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+     n = 0;                                  n = 0;
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+     for (i = 0; i < 10; i++)          =>    for (i = 0; i < 10; i++)
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+       n += atomic_mb_read(&a[i]);             n += atomic_read(&a[i]);
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+                                             smp_rmb();
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+
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+  Similarly, atomic_mb_set() can be transformed as follows:
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+  smp_mb():
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+
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+                                             smp_wmb();
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+     for (i = 0; i < 10; i++)          =>    for (i = 0; i < 10; i++)
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+       atomic_mb_set(&a[i], false);            atomic_set(&a[i], false);
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+                                             smp_mb();
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+
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+
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+The two tricks can be combined.  In this case, splitting a loop in
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+two lets you hoist the barriers out of the loops _and_ eliminate the
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+expensive smp_mb():
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+
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+                                             smp_wmb();
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+     for (i = 0; i < 10; i++) {        =>    for (i = 0; i < 10; i++)
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+       atomic_mb_set(&a[i], false);            atomic_set(&a[i], false);
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+       atomic_mb_set(&b[i], false);          smb_wmb();
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+     }                                       for (i = 0; i < 10; i++)
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+                                               atomic_set(&a[i], false);
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+                                             smp_mb();
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+
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+  The other thread can still use atomic_mb_read()/atomic_mb_set()
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+
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+
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+Memory barrier pairing
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+----------------------
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+
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+A useful rule of thumb is that memory barriers should always, or almost
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+always, be paired with another barrier.  In the case of QEMU, however,
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+note that the other barrier may actually be in a driver that runs in
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+the guest!
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+
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+For the purposes of pairing, smp_read_barrier_depends() and smp_rmb()
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+both count as read barriers.  A read barriers shall pair with a write
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+barrier or a full barrier; a write barrier shall pair with a read
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+barrier or a full barrier.  A full barrier can pair with anything.
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+For example:
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+
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+        thread 1             thread 2
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+        ===============      ===============
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+        a = 1;
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+        smp_wmb();
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+        b = 2;               x = b;
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+                             smp_rmb();
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+                             y = a;
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+
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+Note that the "writing" thread are accessing the variables in the
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+opposite order as the "reading" thread.  This is expected: stores
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+before the write barrier will normally match the loads after the
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+read barrier, and vice versa.  The same is true for more than 2
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+access and for data dependency barriers:
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+
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+        thread 1             thread 2
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+        ===============      ===============
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+        b[2] = 1;
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+        smp_wmb();
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+        x->i = 2;
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+        smp_wmb();
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+        a = x;               x = a;
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+                             smp_read_barrier_depends();
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+                             y = x->i;
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+                             smp_read_barrier_depends();
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+                             z = b[y];
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+
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+smp_wmb() also pairs with atomic_mb_read(), and smp_rmb() also pairs
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+with atomic_mb_set().
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+
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+
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+COMPARISON WITH LINUX KERNEL MEMORY BARRIERS
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+============================================
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+
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+Here is a list of differences between Linux kernel atomic operations
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+and memory barriers, and the equivalents in QEMU:
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+
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+- atomic operations in Linux are always on a 32-bit int type and
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+  use a boxed atomic_t type; atomic operations in QEMU are polymorphic
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+  and use normal C types.
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+
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+- atomic_read and atomic_set in Linux give no guarantee at all;
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+  atomic_read and atomic_set in QEMU include a compiler barrier
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+  (similar to the ACCESS_ONCE macro in Linux).
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+
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+- most atomic read-modify-write operations in Linux return void;
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+  in QEMU, all of them return the old value of the variable.
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+
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+- different atomic read-modify-write operations in Linux imply
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+  a different set of memory barriers; in QEMU, all of them enforce
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+  sequential consistency, which means they imply full memory barriers
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+  before and after the operation.
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+
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+- Linux does not have an equivalent of atomic_mb_read() and
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+  atomic_mb_set().  In particular, note that set_mb() is a little
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+  weaker than atomic_mb_set().
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+
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+
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+SOURCES
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+=======
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+
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+* Documentation/memory-barriers.txt from the Linux kernel
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+
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+* "The JSR-133 Cookbook for Compiler Writers", available at
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+  http://g.oswego.edu/dl/jmm/cookbook.html
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diff --git a/hw/display/qxl.c b/hw/display/qxl.c
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index ea985d2..830b3c5 100644
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--- a/hw/display/qxl.c
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+++ b/hw/display/qxl.c
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@@ -23,6 +23,7 @@
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 #include "qemu-common.h"
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 #include "qemu/timer.h"
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 #include "qemu/queue.h"
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+#include "qemu/atomic.h"
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 #include "monitor/monitor.h"
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 #include "sysemu/sysemu.h"
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 #include "trace.h"
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@@ -1726,7 +1727,7 @@ static void qxl_send_events(PCIQXLDevice *d, uint32_t events)
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         trace_qxl_send_events_vm_stopped(d->id, events);
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         return;
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     }
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-    old_pending = __sync_fetch_and_or(&d->ram->int_pending, le_events);
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+    old_pending = atomic_fetch_or(&d->ram->int_pending, le_events);
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     if ((old_pending & le_events) == le_events) {
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         return;
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     }
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diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
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index 0dabf26..54aa569 100644
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--- a/hw/virtio/vhost.c
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+++ b/hw/virtio/vhost.c
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@@ -16,6 +16,7 @@
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 #include <sys/ioctl.h>
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 #include "hw/virtio/vhost.h"
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 #include "hw/hw.h"
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+#include "qemu/atomic.h"
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 #include "qemu/range.h"
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 #include <linux/vhost.h>
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 #include "exec/address-spaces.h"
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@@ -47,11 +48,9 @@ static void vhost_dev_sync_region(struct vhost_dev *dev,
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             addr += VHOST_LOG_CHUNK;
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             continue;
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         }
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-        /* Data must be read atomically. We don't really
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-         * need the barrier semantics of __sync
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-         * builtins, but it's easier to use them than
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-         * roll our own. */
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-        log = __sync_fetch_and_and(from, 0);
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+        /* Data must be read atomically. We don't really need barrier semantics
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+         * but it's easier to use atomic_* than roll our own. */
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+        log = atomic_xchg(from, 0);
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         while ((bit = sizeof(log) > sizeof(int) ?
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                 ffsll(log) : ffs(log))) {
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             hwaddr page_addr;
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diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
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index 10becb6..0aa8913 100644
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--- a/include/qemu/atomic.h
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+++ b/include/qemu/atomic.h
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@@ -1,68 +1,202 @@
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-#ifndef __QEMU_BARRIER_H
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-#define __QEMU_BARRIER_H 1
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+/*
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+ * Simple interface for atomic operations.
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+ *
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+ * Copyright (C) 2013 Red Hat, Inc.
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+ *
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+ * Author: Paolo Bonzini <pbonzini@redhat.com>
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+ *
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+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
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+ * See the COPYING file in the top-level directory.
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+ *
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+ */
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-/* Compiler barrier */
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-#define barrier()   asm volatile("" ::: "memory")
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+#ifndef __QEMU_ATOMIC_H
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+#define __QEMU_ATOMIC_H 1
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-#if defined(__i386__)
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+#include "qemu/compiler.h"
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-#include "qemu/compiler.h"        /* QEMU_GNUC_PREREQ */
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+/* For C11 atomic ops */
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-/*
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- * Because of the strongly ordered x86 storage model, wmb() and rmb() are nops
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- * on x86(well, a compiler barrier only).  Well, at least as long as
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- * qemu doesn't do accesses to write-combining memory or non-temporal
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- * load/stores from C code.
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- */
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-#define smp_wmb()   barrier()
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-#define smp_rmb()   barrier()
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+/* Compiler barrier */
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+#define barrier()   ({ asm volatile("" ::: "memory"); (void)0; })
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+
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+#ifndef __ATOMIC_RELAXED
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 /*
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- * We use GCC builtin if it's available, as that can use
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- * mfence on 32 bit as well, e.g. if built with -march=pentium-m.
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- * However, on i386, there seem to be known bugs as recently as 4.3.
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- * */
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-#if QEMU_GNUC_PREREQ(4, 4)
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-#define smp_mb() __sync_synchronize()
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+ * We use GCC builtin if it's available, as that can use mfence on
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+ * 32-bit as well, e.g. if built with -march=pentium-m. However, on
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+ * i386 the spec is buggy, and the implementation followed it until
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+ * 4.3 (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36793).
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+ */
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+#if defined(__i386__) || defined(__x86_64__)
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+#if !QEMU_GNUC_PREREQ(4, 4)
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+#if defined __x86_64__
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+#define smp_mb()    ({ asm volatile("mfence" ::: "memory"); (void)0; })
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 #else
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-#define smp_mb() asm volatile("lock; addl $0,0(%%esp) " ::: "memory")
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+#define smp_mb()    ({ asm volatile("lock; addl $0,0(%%esp) " ::: "memory"); (void)0; })
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+#endif
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+#endif
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+#endif
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+
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+
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+#ifdef __alpha__
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+#define smp_read_barrier_depends()   asm volatile("mb":::"memory")
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 #endif
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-#elif defined(__x86_64__)
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+#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__)
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+/*
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+ * Because of the strongly ordered storage model, wmb() and rmb() are nops
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+ * here (a compiler barrier only).  QEMU doesn't do accesses to write-combining
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+ * qemu memory or non-temporal load/stores from C code.
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+ */
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 #define smp_wmb()   barrier()
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 #define smp_rmb()   barrier()
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-#define smp_mb() asm volatile("mfence" ::: "memory")
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+
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+/*
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+ * __sync_lock_test_and_set() is documented to be an acquire barrier only,
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+ * but it is a full barrier at the hardware level.  Add a compiler barrier
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+ * to make it a full barrier also at the compiler level.
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+ */
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+#define atomic_xchg(ptr, i)    (barrier(), __sync_lock_test_and_set(ptr, i))
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+
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+/*
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+ * Load/store with Java volatile semantics.
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+ */
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+#define atomic_mb_set(ptr, i)  ((void)atomic_xchg(ptr, i))
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 #elif defined(_ARCH_PPC)
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 /*
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  * We use an eieio() for wmb() on powerpc.  This assumes we don't
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  * need to order cacheable and non-cacheable stores with respect to
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- * each other
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+ * each other.
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+ *
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+ * smp_mb has the same problem as on x86 for not-very-new GCC
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+ * (http://patchwork.ozlabs.org/patch/126184/, Nov 2011).
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  */
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-#define smp_wmb()   asm volatile("eieio" ::: "memory")
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-
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+#define smp_wmb()   ({ asm volatile("eieio" ::: "memory"); (void)0; })
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 #if defined(__powerpc64__)
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-#define smp_rmb()   asm volatile("lwsync" ::: "memory")
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+#define smp_rmb()   ({ asm volatile("lwsync" ::: "memory"); (void)0; })
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 #else
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-#define smp_rmb()   asm volatile("sync" ::: "memory")
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+#define smp_rmb()   ({ asm volatile("sync" ::: "memory"); (void)0; })
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 #endif
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+#define smp_mb()    ({ asm volatile("sync" ::: "memory"); (void)0; })
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-#define smp_mb()   asm volatile("sync" ::: "memory")
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+#endif /* _ARCH_PPC */
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-#else
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+#endif /* C11 atomics */
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 /*
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  * For (host) platforms we don't have explicit barrier definitions
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  * for, we use the gcc __sync_synchronize() primitive to generate a
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  * full barrier.  This should be safe on all platforms, though it may
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- * be overkill for wmb() and rmb().
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+ * be overkill for smp_wmb() and smp_rmb().
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  */
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+#ifndef smp_mb
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+#define smp_mb()    __sync_synchronize()
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+#endif
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+
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+#ifndef smp_wmb
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+#ifdef __ATOMIC_RELEASE
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+#define smp_wmb()   __atomic_thread_fence(__ATOMIC_RELEASE)
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+#else
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 #define smp_wmb()   __sync_synchronize()
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-#define smp_mb()   __sync_synchronize()
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+#endif
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+#endif
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+
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+#ifndef smp_rmb
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+#ifdef __ATOMIC_ACQUIRE
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+#define smp_rmb()   __atomic_thread_fence(__ATOMIC_ACQUIRE)
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+#else
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 #define smp_rmb()   __sync_synchronize()
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+#endif
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+#endif
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+
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+#ifndef smp_read_barrier_depends
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+#ifdef __ATOMIC_CONSUME
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+#define smp_read_barrier_depends()   __atomic_thread_fence(__ATOMIC_CONSUME)
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+#else
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+#define smp_read_barrier_depends()   barrier()
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+#endif
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+#endif
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+#ifndef atomic_read
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+#define atomic_read(ptr)       (*(__typeof__(*ptr) *volatile) (ptr))
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 #endif
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+#ifndef atomic_set
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+#define atomic_set(ptr, i)     ((*(__typeof__(*ptr) *volatile) (ptr)) = (i))
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+#endif
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+
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+/* These have the same semantics as Java volatile variables.
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+ * See http://gee.cs.oswego.edu/dl/jmm/cookbook.html:
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+ * "1. Issue a StoreStore barrier (wmb) before each volatile store."
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+ *  2. Issue a StoreLoad barrier after each volatile store.
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+ *     Note that you could instead issue one before each volatile load, but
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+ *     this would be slower for typical programs using volatiles in which
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+ *     reads greatly outnumber writes. Alternatively, if available, you
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+ *     can implement volatile store as an atomic instruction (for example
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+ *     XCHG on x86) and omit the barrier. This may be more efficient if
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+ *     atomic instructions are cheaper than StoreLoad barriers.
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+ *  3. Issue LoadLoad and LoadStore barriers after each volatile load."
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+ *
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+ * If you prefer to think in terms of "pairing" of memory barriers,
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+ * an atomic_mb_read pairs with an atomic_mb_set.
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+ *
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+ * And for the few ia64 lovers that exist, an atomic_mb_read is a ld.acq,
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+ * while an atomic_mb_set is a st.rel followed by a memory barrier.
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+ *
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+ * These are a bit weaker than __atomic_load/store with __ATOMIC_SEQ_CST
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+ * (see docs/atomics.txt), and I'm not sure that __ATOMIC_ACQ_REL is enough.
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+ * Just always use the barriers manually by the rules above.
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+ */
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+#ifndef atomic_mb_read
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+#define atomic_mb_read(ptr)    ({           \
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+    typeof(*ptr) _val = atomic_read(ptr);   \
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+    smp_rmb();                              \
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+    _val;                                   \
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+})
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+#endif
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+
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+#ifndef atomic_mb_set
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+#define atomic_mb_set(ptr, i)  do {         \
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+    smp_wmb();                              \
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+    atomic_set(ptr, i);                     \
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+    smp_mb();                               \
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+} while (0)
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+#endif
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+
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+#ifndef atomic_xchg
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+#ifdef __ATOMIC_SEQ_CST
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+#define atomic_xchg(ptr, i)    ({                           \
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+    typeof(*ptr) _new = (i), _old;                          \
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+    __atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
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+    _old;                                                   \
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+})
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+#elif defined __clang__
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+#define atomic_xchg(ptr, i)    __sync_exchange(ptr, i)
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+#else
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+/* __sync_lock_test_and_set() is documented to be an acquire barrier only.  */
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+#define atomic_xchg(ptr, i)    (smp_mb(), __sync_lock_test_and_set(ptr, i))
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+#endif
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+#endif
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+
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+/* Provide shorter names for GCC atomic builtins.  */
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+#define atomic_fetch_inc(ptr)  __sync_fetch_and_add(ptr, 1)
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+#define atomic_fetch_dec(ptr)  __sync_fetch_and_add(ptr, -1)
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+#define atomic_fetch_add       __sync_fetch_and_add
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+#define atomic_fetch_sub       __sync_fetch_and_sub
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+#define atomic_fetch_and       __sync_fetch_and_and
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+#define atomic_fetch_or        __sync_fetch_and_or
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+#define atomic_cmpxchg         __sync_val_compare_and_swap
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+
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+/* And even shorter names that return void.  */
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+#define atomic_inc(ptr)        ((void) __sync_fetch_and_add(ptr, 1))
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+#define atomic_dec(ptr)        ((void) __sync_fetch_and_add(ptr, -1))
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+#define atomic_add(ptr, n)     ((void) __sync_fetch_and_add(ptr, n))
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+#define atomic_sub(ptr, n)     ((void) __sync_fetch_and_sub(ptr, n))
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+#define atomic_and(ptr, n)     ((void) __sync_fetch_and_and(ptr, n))
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+#define atomic_or(ptr, n)      ((void) __sync_fetch_and_or(ptr, n))
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+
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 #endif
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diff --git a/migration.c b/migration.c
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index 46c633a..d91e702 100644
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--- a/migration.c
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+++ b/migration.c
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@@ -291,8 +291,7 @@ static void migrate_fd_cleanup(void *opaque)
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 static void migrate_finish_set_state(MigrationState *s, int new_state)
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 {
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-    if (__sync_val_compare_and_swap(&s->state, MIG_STATE_ACTIVE,
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-                                    new_state) == new_state) {
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+    if (atomic_cmpxchg(&s->state, MIG_STATE_ACTIVE, new_state) == new_state) {
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         trace_migrate_set_state(new_state);
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     }
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 }
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diff --git a/tests/test-thread-pool.c b/tests/test-thread-pool.c
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index 22915aa..b62338f 100644
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--- a/tests/test-thread-pool.c
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+++ b/tests/test-thread-pool.c
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@@ -17,15 +17,15 @@ typedef struct {
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 static int worker_cb(void *opaque)
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 {
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     WorkerTestData *data = opaque;
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-    return __sync_fetch_and_add(&data->n, 1);
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+    return atomic_fetch_inc(&data->n);
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 }
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 static int long_cb(void *opaque)
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 {
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     WorkerTestData *data = opaque;
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-    __sync_fetch_and_add(&data->n, 1);
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+    atomic_inc(&data->n);
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     g_usleep(2000000);
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-    __sync_fetch_and_add(&data->n, 1);
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+    atomic_inc(&data->n);
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     return 0;
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 }
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@@ -169,7 +169,7 @@ static void test_cancel(void)
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     /* Cancel the jobs that haven't been started yet.  */
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     num_canceled = 0;
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     for (i = 0; i < 100; i++) {
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-        if (__sync_val_compare_and_swap(&data[i].n, 0, 3) == 0) {
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+        if (atomic_cmpxchg(&data[i].n, 0, 3) == 0) {
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             data[i].ret = -ECANCELED;
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             bdrv_aio_cancel(data[i].aiocb);
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             active--;
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-- 
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1.7.11.7
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