aio_wait_kick: add missing memory barrier

It seems that aio_wait_kick always required a memory barrier
or atomic operation in the caller, but nobody actually
took care of doing it.

Let's put the barrier in the function instead, and pair it
with another one in AIO_WAIT_WHILE. Read aio_wait_kick()
comment for further explanation.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220524173054.12651-1-eesposit@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
index b39eefb38d1607e881b4020e384b644d8849ff1d..54840f8622358c9d8300be827d9d987a2c758625 100644 (file)
--- a/include/block/aio-wait.h
+++ b/include/block/aio-wait.h
@@ -81,6 +81,8 @@ extern AioWait global_aio_wait;
     AioContext *ctx_ = (ctx);                                      \
     /* Increment wait_->num_waiters before evaluating cond. */     \
     qatomic_inc(&wait_->num_waiters);                              \
+    /* Paired with smp_mb in aio_wait_kick(). */                   \
+    smp_mb();                                                      \
     if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
         while ((cond)) {                                           \
             aio_poll(ctx_, true);                                  \

diff --git a/util/aio-wait.c b/util/aio-wait.c
index bdb3d3af22ddbb0b77b52e12b4e9538acebba08e..98c5accd29d0f2999b59e3a898e74ba9a46ce812 100644 (file)
--- a/util/aio-wait.c
+++ b/util/aio-wait.c
@@ -35,7 +35,21 @@ static void dummy_bh_cb(void *opaque)
 
 void aio_wait_kick(void)
 {
-    /* The barrier (or an atomic op) is in the caller.  */
+    /*
+     * Paired with smp_mb in AIO_WAIT_WHILE. Here we have:
+     * write(condition);
+     * aio_wait_kick() {
+     *      smp_mb();
+     *      read(num_waiters);
+     * }
+     *
+     * And in AIO_WAIT_WHILE:
+     * write(num_waiters);
+     * smp_mb();
+     * read(condition);
+     */
+    smp_mb();
+
     if (qatomic_read(&global_aio_wait.num_waiters)) {
         aio_bh_schedule_oneshot(qemu_get_aio_context(), dummy_bh_cb, NULL);
     }