Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task work

On Aug 9, 2021, at 2:48 PM, Olivier Langlois [off-list ref] wrote:

On Sat, 2021-08-07 at 17:13 -0700, Nadav Amit wrote:

quoted

From: Nadav Amit <redacted>

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
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.

thx a lot for this patch!

This explains what I am seeing here:
https://lore.kernel.org/io-uring/4d93d0600e4a9590a48d320c5a7dd4c54d66f095.camel@trillion01.com/ (local)

I was under the impression that task_work_run() was clearing
TIF_NOTIFY_SIGNAL.

your patch made me realize that it does not…

Happy it could help.

Unfortunately, there seems to be yet another issue (unless my code
somehow caused it). It seems that when SQPOLL is used, there are cases
in which we get stuck in io_uring_cancel_sqpoll() when tctx_inflight()
never goes down to zero.

Debugging... (while also trying to make some progress with my code)