Re: [PATCH] aio_wait_kick: add missing memory barrier

[Roman Kagan <rvkagan@yandex-team.ru>]

On Tue, May 24, 2022 at 01:30:54PM -0400, Emanuele Giuseppe Esposito wrote:
> 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>
> ---
>  include/block/aio-wait.h |  2 ++
>  util/aio-wait.c          | 16 +++++++++++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
> index b39eefb38d..54840f8622 100644
> --- 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();                                                      \

IIRC qatomic_inc() ensures sequential consistency, isn't it enough here?

>      if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
>          while ((cond)) {                                           \
>              aio_poll(ctx_, true);                                  \

Roman.