linux-kernel - Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY

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Date:   Sun, 8 Aug 2021 13:55:12 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Nadav Amit <nadav.amit@...il.com>, Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task
 work

On 8/8/21 1:13 AM, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> When using SQPOLL, the submission queue polling thread calls
> task_work_run() to run queued work. However, when work is added with
> TWA_SIGNAL - as done by io_uring itself - the TIF_NOTIFY_SIGNAL remains

static int io_req_task_work_add(struct io_kiocb *req)
{
	...
	notify = (req->ctx->flags & IORING_SETUP_SQPOLL) ? TWA_NONE : TWA_SIGNAL;
	if (!task_work_add(tsk, &tctx->task_work, notify))
	...
}

io_uring doesn't set TIF_NOTIFY_SIGNAL for SQPOLL. But if you see it, I'm
rather curious who does.


> set afterwards and is never cleared.
> 
> Consequently, when the submission queue polling thread checks whether
> signal_pending(), it may always find a pending signal, if
> task_work_add() was ever called before.
> 
> The impact of this bug might be different on different kernel versions.
> It appears that on 5.14 it would only cause unnecessary calculation and
> prevent the polling thread from sleeping. On 5.13, where the bug was
> found, it stops the polling thread from finding newly submitted work.
> 
> Instead of task_work_run(), use tracehook_notify_signal() that clears
> TIF_NOTIFY_SIGNAL. Test for TIF_NOTIFY_SIGNAL in addition to
> current->task_works to avoid a race in which task_works is cleared but
> the TIF_NOTIFY_SIGNAL is set.
> 
> Fixes: 685fe7feedb96 ("io-wq: eliminate the need for a manager thread")
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Pavel Begunkov <asml.silence@...il.com>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>  fs/io_uring.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 5a0fd6bcd318..f39244d35f90 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -78,6 +78,7 @@
>  #include <linux/task_work.h>
>  #include <linux/pagemap.h>
>  #include <linux/io_uring.h>
> +#include <linux/tracehook.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/io_uring.h>
> @@ -2203,9 +2204,9 @@ static inline unsigned int io_put_rw_kbuf(struct io_kiocb *req)
>  
>  static inline bool io_run_task_work(void)
>  {
> -	if (current->task_works) {
> +	if (test_thread_flag(TIF_NOTIFY_SIGNAL) || current->task_works) {
>  		__set_current_state(TASK_RUNNING);
> -		task_work_run();
> +		tracehook_notify_signal();
>  		return true;
>  	}
>  
> 

-- 
Pavel Begunkov