Re: [PATCH 2/5] io_uring: serialize ctx->rings->sq_flags with cmpxchg()

[Pavel Begunkov <asml.silence@gmail.com>]

On 4/22/22 22:42, Jens Axboe wrote:
> Rather than require ctx->completion_lock for ensuring that we don't
> clobber the flags, use try_cmpxchg() instead. This removes the need
> to grab the completion_lock, in preparation for needing to set or
> clear sq_flags when we don't know the status of this lock.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>   fs/io_uring.c | 54 ++++++++++++++++++++++++++++++---------------------
>   1 file changed, 32 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 626bf840bed2..38e58fe4963d 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1999,6 +1999,34 @@ static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
>   		io_cqring_wake(ctx);
>   }
>   
> +static void io_ring_sq_flag_clear(struct io_ring_ctx *ctx, unsigned int flag)
> +{
> +	struct io_rings *rings = ctx->rings;
> +	unsigned int oldf, newf;
> +
> +	do {
> +		oldf = READ_ONCE(rings->sq_flags);
> +
> +		if (!(oldf & flag))
> +			break;
> +		newf = oldf & ~flag;
> +	} while (!try_cmpxchg(&rings->sq_flags, &oldf, newf));
> +}
> +
> +static void io_ring_sq_flag_set(struct io_ring_ctx *ctx, unsigned int flag)
> +{
> +	struct io_rings *rings = ctx->rings;
> +	unsigned int oldf, newf;
> +
> +	do {
> +		oldf = READ_ONCE(rings->sq_flags);
> +
> +		if (oldf & flag)
> +			break;
> +		newf = oldf | flag;
> +	} while (!try_cmpxchg(&rings->sq_flags, &oldf, newf));
> +}

atomic and/or might be a better fit

-- 
Pavel Begunkov