The barrier comes after an atomic increment, so it is enough to use
smp_mb__after_rmw(); this avoids a double barrier on x86 systems.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/block/aio-wait.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
index dd9a7f6461ef..da13357bb8cf 100644
--- a/include/block/aio-wait.h
+++ b/include/block/aio-wait.h
@@ -85,7 +85,7 @@ extern AioWait global_aio_wait;
     /* Increment wait_->num_waiters before evaluating cond. */     \
     qatomic_inc(&wait_->num_waiters);                              \
     /* Paired with smp_mb in aio_wait_kick(). */                   \
-    smp_mb();                                                      \
+    smp_mb__after_rmw();                                           \
     if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
         while ((cond)) {                                           \
             aio_poll(ctx_, true);                                  \
-- 
2.39.1

On 06.03.23 23:33, Paolo Bonzini wrote:
> The barrier comes after an atomic increment, so it is enough to use
> smp_mb__after_rmw(); this avoids a double barrier on x86 systems.
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   include/block/aio-wait.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
> index dd9a7f6461ef..da13357bb8cf 100644
> --- a/include/block/aio-wait.h
> +++ b/include/block/aio-wait.h
> @@ -85,7 +85,7 @@ extern AioWait global_aio_wait;
>       /* Increment wait_->num_waiters before evaluating cond. */     \
>       qatomic_inc(&wait_->num_waiters);                              \
>       /* Paired with smp_mb in aio_wait_kick(). */                   \
> -    smp_mb();                                                      \
> +    smp_mb__after_rmw();                                           \
>       if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
>           while ((cond)) {                                           \
>               aio_poll(ctx_, true);                                  \

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb