Re: [PATCH v3 3/3] io_uring: don't get completion_lock in io_poll_rewait()

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

On 10/25/21 06:38, Xiaoguang Wang wrote:
> In current implementation, if there are not available events,
> io_poll_rewait() just gets completion_lock, and unlocks it in
> io_poll_task_func() or io_async_task_func(), which isn't necessary.
> 
> Change this logic to let io_poll_task_func() or io_async_task_func()
> get the completion_lock lock.
> 
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>   fs/io_uring.c | 58 ++++++++++++++++++++++++++--------------------------------
>   1 file changed, 26 insertions(+), 32 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index e4c779dac953..41ff8fdafe55 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -5248,10 +5248,7 @@ static inline int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *pol
>   }
>   
>   static bool io_poll_rewait(struct io_kiocb *req, struct io_poll_iocb *poll)
> -	__acquires(&req->ctx->completion_lock)
>   {
> -	struct io_ring_ctx *ctx = req->ctx;
> -
>   	/* req->task == current here, checking PF_EXITING is safe */
>   	if (unlikely(req->task->flags & PF_EXITING))
>   		WRITE_ONCE(poll->canceled, true);
> @@ -5262,7 +5259,6 @@ static bool io_poll_rewait(struct io_kiocb *req, struct io_poll_iocb *poll)
>   		req->result = vfs_poll(req->file, &pt) & poll->events;
>   	}
>   
> -	spin_lock(&ctx->completion_lock);

Don't remember poll sync too well but this was synchronising with the
final section of __io_arm_poll_handler(), and I'm afraid it may go
completely loose with races.


>   	if (!req->result && !READ_ONCE(poll->canceled)) {
>   		if (req->opcode == IORING_OP_POLL_ADD)
>   			WRITE_ONCE(poll->active, true);
-- 
Pavel Begunkov