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From: Pavel Begunkov <asml.silence@gmail.com>
To: Nadav Amit <nadav.amit@gmail.com>,
	Olivier Langlois <olivier@trillion01.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	io-uring@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task work
Date: Tue, 10 Aug 2021 22:32:59 +0100	[thread overview]
Message-ID: <bbd25a42-eac0-a8f9-0e54-3c8c8e9894fd@gmail.com> (raw)
In-Reply-To: <A4DC14BA-74CA-41DB-BE08-D7B693C11AE0@gmail.com>

On 8/10/21 9:28 AM, Nadav Amit wrote:
>> On Aug 9, 2021, at 2:48 PM, Olivier Langlois <olivier@trillion01.com> wrote:
>> On Sat, 2021-08-07 at 17:13 -0700, Nadav Amit wrote:
>>> From: Nadav Amit <namit@vmware.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
>>> 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/
>>
>> 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)

It's most likely because a request has been lost (mis-refcounted).
Let us know if you need any help. Would be great to solve it for 5.14.
quick tips: 

1) if not already, try out Jens' 5.14 branch
git://git.kernel.dk/linux-block io_uring-5.14

2) try to characterise the io_uring use pattern. Poll requests?
Read/write requests? Send/recv? Filesystem vs bdev vs sockets?

If easily reproducible, you can match io_alloc_req() with it
getting into io_dismantle_req();

-- 
Pavel Begunkov

  parent reply	other threads:[~2021-08-10 21:33 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-08  0:13 [PATCH 0/2] io_uring: bug fixes Nadav Amit
2021-08-08  0:13 ` [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task work Nadav Amit
2021-08-08 12:55   ` Pavel Begunkov
2021-08-08 17:31     ` Nadav Amit
2021-08-09  4:07       ` Hao Xu
2021-08-09  4:50         ` Nadav Amit
2021-08-09 10:35           ` Pavel Begunkov
2021-08-09 10:18       ` Pavel Begunkov
2021-08-09 21:48   ` Olivier Langlois
2021-08-10  8:28     ` Nadav Amit
2021-08-10 13:33       ` Olivier Langlois
2021-08-10 21:32       ` Pavel Begunkov [this message]
2021-08-11  2:33         ` Nadav Amit
2021-08-11  2:51           ` Jens Axboe
2021-08-11  5:40             ` I/O cancellation in io-uring (was: io_uring: clear TIF_NOTIFY_SIGNAL ...) Nadav Amit
2021-08-08  0:13 ` [PATCH 2/2] io_uring: Use WRITE_ONCE() when writing to sq_flags Nadav Amit
2021-08-09 13:53 ` [PATCH 0/2] io_uring: bug fixes Jens Axboe

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