Re: [PATCH] aio_wait_kick: add missing memory barrier
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
> Signed-off-by: Emanuele Giuseppe Esposito
> ---
> 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.
Re: [PATCH] aio_wait_kick: add missing memory barrier
On 6/4/22 14:51, Roman Kagan wrote: 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 Signed-off-by: Emanuele Giuseppe Esposito --- 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? Nope, it only ensures sequential consistency with other SEQ_CST operations, i.e. not with qatomic_read or qatomic_set. :( The smp_mb() is needed on ARM, in particular. Paolo
