From aa61e4c437d29a791ea09a01f7230231f1e53356 Mon Sep 17 00:00:00 2001
From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Thu, 9 Mar 2023 08:24:36 -0500
Subject: [PATCH 05/12] qemu-thread-posix: cleanup, fix, document QemuEvent

RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
RH-MergeRequest: 158: qatomic: add smp_mb__before/after_rmw()
RH-Bugzilla: 2175660
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Acked-by: Eric Auger <eric.auger@redhat.com>
RH-Commit: [2/9] c3bdf75f884e137c667316aaac96bb4a0b9ec2d9 (eesposit/qemu-kvm)

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175660

commit 9586a1329f5dce6c1d7f4de53cf0536644d7e593
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Thu Mar 2 11:19:52 2023 +0100

    qemu-thread-posix: cleanup, fix, document QemuEvent

    QemuEvent is currently broken on ARM due to missing memory barriers
    after qatomic_*().  Apart from adding the memory barrier, a closer look
    reveals some unpaired memory barriers too.  Document more clearly what
    is going on.

    Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
    Reviewed-by: David Hildenbrand <david@redhat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 util/qemu-thread-posix.c | 69 ++++++++++++++++++++++++++++------------
 1 file changed, 49 insertions(+), 20 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index bae938c670..cc74f4ede0 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -379,13 +379,21 @@ void qemu_event_destroy(QemuEvent *ev)
 
 void qemu_event_set(QemuEvent *ev)
 {
-    /* qemu_event_set has release semantics, but because it *loads*
+    assert(ev->initialized);
+
+    /*
+     * Pairs with both qemu_event_reset() and qemu_event_wait().
+     *
+     * qemu_event_set has release semantics, but because it *loads*
      * ev->value we need a full memory barrier here.
      */
-    assert(ev->initialized);
     smp_mb();
     if (qatomic_read(&ev->value) != EV_SET) {
-        if (qatomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
+        int old = qatomic_xchg(&ev->value, EV_SET);
+
+        /* Pairs with memory barrier in kernel futex_wait system call.  */
+        smp_mb__after_rmw();
+        if (old == EV_BUSY) {
             /* There were waiters, wake them up.  */
             qemu_futex_wake(ev, INT_MAX);
         }
@@ -394,18 +402,19 @@ void qemu_event_set(QemuEvent *ev)
 
 void qemu_event_reset(QemuEvent *ev)
 {
-    unsigned value;
-
     assert(ev->initialized);
-    value = qatomic_read(&ev->value);
-    smp_mb_acquire();
-    if (value == EV_SET) {
-        /*
-         * If there was a concurrent reset (or even reset+wait),
-         * do nothing.  Otherwise change EV_SET->EV_FREE.
-         */
-        qatomic_or(&ev->value, EV_FREE);
-    }
+
+    /*
+     * If there was a concurrent reset (or even reset+wait),
+     * do nothing.  Otherwise change EV_SET->EV_FREE.
+     */
+    qatomic_or(&ev->value, EV_FREE);
+
+    /*
+     * Order reset before checking the condition in the caller.
+     * Pairs with the first memory barrier in qemu_event_set().
+     */
+    smp_mb__after_rmw();
 }
 
 void qemu_event_wait(QemuEvent *ev)
@@ -413,20 +422,40 @@ void qemu_event_wait(QemuEvent *ev)
     unsigned value;
 
     assert(ev->initialized);
-    value = qatomic_read(&ev->value);
-    smp_mb_acquire();
+
+    /*
+     * qemu_event_wait must synchronize with qemu_event_set even if it does
+     * not go down the slow path, so this load-acquire is needed that
+     * synchronizes with the first memory barrier in qemu_event_set().
+     *
+     * If we do go down the slow path, there is no requirement at all: we
+     * might miss a qemu_event_set() here but ultimately the memory barrier in
+     * qemu_futex_wait() will ensure the check is done correctly.
+     */
+    value = qatomic_load_acquire(&ev->value);
     if (value != EV_SET) {
         if (value == EV_FREE) {
             /*
-             * Leave the event reset and tell qemu_event_set that there
-             * are waiters.  No need to retry, because there cannot be
-             * a concurrent busy->free transition.  After the CAS, the
-             * event will be either set or busy.
+             * Leave the event reset and tell qemu_event_set that there are
+             * waiters.  No need to retry, because there cannot be a concurrent
+             * busy->free transition.  After the CAS, the event will be either
+             * set or busy.
+             *
+             * This cmpxchg doesn't have particular ordering requirements if it
+             * succeeds (moving the store earlier can only cause qemu_event_set()
+             * to issue _more_ wakeups), the failing case needs acquire semantics
+             * like the load above.
              */
             if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
                 return;
             }
         }
+
+        /*
+         * This is the final check for a concurrent set, so it does need
+         * a smp_mb() pairing with the second barrier of qemu_event_set().
+         * The barrier is inside the FUTEX_WAIT system call.
+         */
         qemu_futex_wait(ev, EV_BUSY);
     }
 }
-- 
2.39.1