linux-kernel - Re: [PATCH 1/2 v2] io_uring: Fix race condition when sqp thread goes to sleep

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Date:   Tue, 22 Jun 2021 21:45:00 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Olivier Langlois <olivier@...llion01.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] io_uring: Fix race condition when sqp thread goes
 to sleep

On 6/22/21 7:55 PM, Olivier Langlois wrote:
> If an asynchronous completion happens before the task is preparing
> itself to wait and set its state to TASK_INTERRUPTIBLE, the completion
> will not wake up the sqp thread.
> 
> Signed-off-by: Olivier Langlois <olivier@...llion01.com>
> ---
>  fs/io_uring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index fc8637f591a6..02f789e07d4c 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6902,7 +6902,7 @@ static int io_sq_thread(void *data)
>  		}
>  
>  		prepare_to_wait(&sqd->wait, &wait, TASK_INTERRUPTIBLE);
> -		if (!io_sqd_events_pending(sqd)) {
> +		if (!io_sqd_events_pending(sqd) && !current->task_works) {

Agree that it should be here, but we also lack a good enough
task_work_run() around, and that may send the task burn CPU
for a while in some cases. Let's do

if (!io_sqd_events_pending(sqd) && !io_run_task_work())
   ...

fwiw, no need to worry about TASK_INTERRUPTIBLE as
io_run_task_work() sets it to TASK_RUNNING.

>  			needs_sched = true;
>  			list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
>  				io_ring_set_wakeup_flag(ctx);
> 

-- 
Pavel Begunkov