linux-kernel - Re: [PATCH] io_uring/sqpoll: annotate data race for access in debug check

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Message-ID: <2b5dd8cc-c9d0-4524-b2c0-e99f760a2e36@gmail.com>
Date: Wed, 8 Jan 2025 23:42:59 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: lizetao <lizetao1@...wei.com>
Cc: Jens Axboe <axboe@...nel.dk>, Pavel Begunkov <asml.silence@...il.com>,
 "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
 "syzbot+5988142e8a69a67b1418@...kaller.appspotmail.com"
 <syzbot+5988142e8a69a67b1418@...kaller.appspotmail.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] io_uring/sqpoll: annotate data race for access in debug
 check

On 1/8/25 23:24, lizetao wrote:
> Hi,
> 
>> -----Original Message-----
>> From: Bui Quang Minh <minhquangbui99@...il.com>
>> Sent: Wednesday, January 8, 2025 11:11 PM
>> To: linux-kernel@...r.kernel.org
>> Cc: Bui Quang Minh <minhquangbui99@...il.com>; Jens Axboe
>> <axboe@...nel.dk>; Pavel Begunkov <asml.silence@...il.com>; io-
>> uring@...r.kernel.org;
>> syzbot+5988142e8a69a67b1418@...kaller.appspotmail.com
>> Subject: [PATCH] io_uring/sqpoll: annotate data race for access in debug check
>>
>> sqd->thread must only be access while holding sqd->lock. In
>> io_sq_thread_stop, the sqd->thread access to wake up the sq thread is placed
>> while holding sqd->lock, but the access in debug check is not. As this access if
>> for debug check only, we can safely ignore the data race here. So we annotate
>> this access with data_race to silence KCSAN.
>>
>> Reported-by: syzbot+5988142e8a69a67b1418@...kaller.appspotmail.com
>> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
>> ---
>>   io_uring/sqpoll.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c index
>> 9e5bd79fd2b5..2088c56dbaa0 100644
>> --- a/io_uring/sqpoll.c
>> +++ b/io_uring/sqpoll.c
>> @@ -57,7 +57,7 @@ void io_sq_thread_park(struct io_sq_data *sqd)
>>
>>   void io_sq_thread_stop(struct io_sq_data *sqd)  {
>> -	WARN_ON_ONCE(sqd->thread == current);
>> +	WARN_ON_ONCE(data_race(sqd->thread) == current);
>>   	WARN_ON_ONCE(test_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd-
>>> state));
>>
>>   	set_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd->state);
>> --
>> 2.43.0
>>
> 
> The modification of this patch itself is fine, but there are two other things I need to confirm.
> 1、Does the io_uring_cancel_generic() require the same modification?

I think yes, there is another syzbot's bug report on data race on the 
io_uring_cancel_generic I'm currently looking at. Here is the link: 
https://syzkaller.appspot.com/bug?extid=3c750be01dab672c513d

> 2、It is not holding sqd->lock in io_req_normal_work_add(), is it safe?

This is a valid point, I think we should add lock here too. I will try 
to write a proof-of-concept to validate this.

Thanks,
Quang Minh.