Partial backport:
Skipped elide.h since rw-locks are not backported.
commit ca6e601a9d4a72b3699cca15bad12ac1716bf49a
Author: Torvald Riegel <triegel@redhat.com>
Date: Wed Nov 30 17:53:11 2016 +0100
Use C11-like atomics instead of plain memory accesses in x86 lock elision.
This uses atomic operations to access lock elision metadata that is accessed
concurrently (ie, adapt_count fields). The size of the data is less than a
word but accessed only with atomic loads and stores; therefore, we add
support for shorter-size atomic load and stores too.
* include/atomic.h (__atomic_check_size_ls): New.
(atomic_load_relaxed, atomic_load_acquire, atomic_store_relaxed,
atomic_store_release): Use it.
* sysdeps/x86/elide.h (ACCESS_ONCE): Remove.
(elision_adapt, ELIDE_LOCK): Use atomics.
* sysdeps/unix/sysv/linux/x86/elision-lock.c (__lll_lock_elision): Use
atomics and improve code comments.
* sysdeps/unix/sysv/linux/x86/elision-trylock.c
(__lll_trylock_elision): Likewise.
Index: glibc-2.17-c758a686/include/atomic.h
===================================================================
--- glibc-2.17-c758a686.orig/include/atomic.h
+++ glibc-2.17-c758a686/include/atomic.h
@@ -567,6 +567,20 @@ void __atomic_link_error (void);
if (sizeof (*mem) != 4) \
__atomic_link_error ();
# endif
+/* We additionally provide 8b and 16b atomic loads and stores; we do not yet
+ need other atomic operations of such sizes, and restricting the support to
+ loads and stores makes this easier for archs that do not have native
+ support for atomic operations to less-than-word-sized data. */
+# if __HAVE_64B_ATOMICS == 1
+# define __atomic_check_size_ls(mem) \
+ if ((sizeof (*mem) != 1) && (sizeof (*mem) != 2) && (sizeof (*mem) != 4) \
+ && (sizeof (*mem) != 8)) \
+ __atomic_link_error ();
+# else
+# define __atomic_check_size_ls(mem) \
+ if ((sizeof (*mem) != 1) && (sizeof (*mem) != 2) && sizeof (*mem) != 4) \
+ __atomic_link_error ();
+# endif
# define atomic_thread_fence_acquire() \
__atomic_thread_fence (__ATOMIC_ACQUIRE)
@@ -576,18 +590,20 @@ void __atomic_link_error (void);
__atomic_thread_fence (__ATOMIC_SEQ_CST)
# define atomic_load_relaxed(mem) \
- ({ __atomic_check_size((mem)); __atomic_load_n ((mem), __ATOMIC_RELAXED); })
+ ({ __atomic_check_size_ls((mem)); \
+ __atomic_load_n ((mem), __ATOMIC_RELAXED); })
# define atomic_load_acquire(mem) \
- ({ __atomic_check_size((mem)); __atomic_load_n ((mem), __ATOMIC_ACQUIRE); })
+ ({ __atomic_check_size_ls((mem)); \
+ __atomic_load_n ((mem), __ATOMIC_ACQUIRE); })
# define atomic_store_relaxed(mem, val) \
do { \
- __atomic_check_size((mem)); \
+ __atomic_check_size_ls((mem)); \
__atomic_store_n ((mem), (val), __ATOMIC_RELAXED); \
} while (0)
# define atomic_store_release(mem, val) \
do { \
- __atomic_check_size((mem)); \
+ __atomic_check_size_ls((mem)); \
__atomic_store_n ((mem), (val), __ATOMIC_RELEASE); \
} while (0)
Index: glibc-2.17-c758a686/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
===================================================================
--- glibc-2.17-c758a686.orig/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
+++ glibc-2.17-c758a686/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
@@ -44,7 +44,13 @@
int
__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
{
- if (*adapt_count <= 0)
+ /* adapt_count can be accessed concurrently; these accesses can be both
+ inside of transactions (if critical sections are nested and the outer
+ critical section uses lock elision) and outside of transactions. Thus,
+ we need to use atomic accesses to avoid data races. However, the
+ value of adapt_count is just a hint, so relaxed MO accesses are
+ sufficient. */
+ if (atomic_load_relaxed (adapt_count) <= 0)
{
unsigned status;
int try_xbegin;
@@ -70,15 +76,20 @@ __lll_lock_elision (int *futex, short *a
&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
{
/* Right now we skip here. Better would be to wait a bit
- and retry. This likely needs some spinning. */
- if (*adapt_count != aconf.skip_lock_busy)
- *adapt_count = aconf.skip_lock_busy;
+ and retry. This likely needs some spinning. See
+ above for why relaxed MO is sufficient. */
+ if (atomic_load_relaxed (adapt_count)
+ != aconf.skip_lock_busy)
+ atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
}
/* Internal abort. There is no chance for retry.
Use the normal locking and next time use lock.
- Be careful to avoid writing to the lock. */
- else if (*adapt_count != aconf.skip_lock_internal_abort)
- *adapt_count = aconf.skip_lock_internal_abort;
+ Be careful to avoid writing to the lock. See above for why
+ relaxed MO is sufficient. */
+ else if (atomic_load_relaxed (adapt_count)
+ != aconf.skip_lock_internal_abort)
+ atomic_store_relaxed (adapt_count,
+ aconf.skip_lock_internal_abort);
break;
}
}
@@ -87,7 +98,8 @@ __lll_lock_elision (int *futex, short *a
{
/* Use a normal lock until the threshold counter runs out.
Lost updates possible. */
- (*adapt_count)--;
+ atomic_store_relaxed (adapt_count,
+ atomic_load_relaxed (adapt_count) - 1);
}
/* Use a normal lock here. */
Index: glibc-2.17-c758a686/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
===================================================================
--- glibc-2.17-c758a686.orig/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
+++ glibc-2.17-c758a686/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
@@ -36,8 +36,10 @@ __lll_trylock_elision (int *futex, short
return an error. */
_xabort (_ABORT_NESTED_TRYLOCK);
- /* Only try a transaction if it's worth it. */
- if (*adapt_count <= 0)
+ /* Only try a transaction if it's worth it. See __lll_lock_elision for
+ why we need atomic accesses. Relaxed MO is sufficient because this is
+ just a hint. */
+ if (atomic_load_relaxed (adapt_count) <= 0)
{
unsigned status;
@@ -55,16 +57,18 @@ __lll_trylock_elision (int *futex, short
if (!(status & _XABORT_RETRY))
{
/* Internal abort. No chance for retry. For future
- locks don't try speculation for some time. */
- if (*adapt_count != aconf.skip_trylock_internal_abort)
- *adapt_count = aconf.skip_trylock_internal_abort;
+ locks don't try speculation for some time. See above for MO. */
+ if (atomic_load_relaxed (adapt_count)
+ != aconf.skip_lock_internal_abort)
+ atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort);
}
/* Could do some retries here. */
}
else
{
- /* Lost updates are possible, but harmless. */
- (*adapt_count)--;
+ /* Lost updates are possible but harmless (see above). */
+ atomic_store_relaxed (adapt_count,
+ atomic_load_relaxed (adapt_count) - 1);
}
return lll_trylock (*futex);