Re: [PATCH 03/11] fs: aio: Use acquire/release for ring->tail publication

[Jan Kara]

On Tue 25-08-26 17:54:14, Jinjie Ruan wrote:
> Replace the smp_wmb() + WRITE_ONCE(ring->tail) and READ_ONCE(ring->tail)
> + smp_rmb() barrier pair with smp_store_release()/smp_load_acquire()
> on `ring->tail`.
> 
> This expresses the publish/subscribe pattern more clearly and allows
> architectures with native acquire/release instructions (e.g. arm64's
> STLR/LDAR) to avoid the cost of full one-way barriers (DMB ISHST/ISHLD).
> 
> The release ensures event data written before updating ring->tail is
> visible to readers that observe the new tail value via acquire, which
> is exactly the ordering the barrier pair provided.
> 
> No functional change intended.
> 
> Assisted-by: DeepSeek:DeepSeek-V3
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

The patch looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

Just one nit below:

> diff --git a/fs/aio.c b/fs/aio.c
> index d78acc69f487..4413c82688cc 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1206,13 +1206,12 @@ static void aio_complete(struct aio_kiocb *iocb)
>  	/* after flagging the request as done, we
>  	 * must never even look at it again
>  	 */
> -	smp_wmb();	/* make event visible before updating tail */
> -
> -	ctx->tail = tail;
> +	WRITE_ONCE(ctx->tail, tail);

I don't thing ctx->tail is accessed anywhere outside of completion_lock so
I think this WRITE_ONCE is pointless.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR