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From: Hao Xu <[email protected]>
To: Pavel Begunkov <[email protected]>, Jens Axboe <[email protected]>
Cc: [email protected], Joseph Qi <[email protected]>
Subject: Re: [PATCH 1/2] io_uring: fix no lock protection for ctx->cq_extra
Date: Fri, 26 Nov 2021 11:29:53 +0800	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

在 2021/11/25 下午10:30, Pavel Begunkov 写道:
> On 11/25/21 09:21, Hao Xu wrote:
>> ctx->cq_extra should be protected by completion lock so that the
>> req_need_defer() does the right check.
>>
>> Cc: [email protected]
>> Signed-off-by: Hao Xu <[email protected]>
>> ---
>>   fs/io_uring.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index f666a0e7f5e8..ae9534382b26 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -6537,12 +6537,15 @@ static __cold void io_drain_req(struct 
>> io_kiocb *req)
>>       u32 seq = io_get_sequence(req);
>>       /* Still need defer if there is pending req in defer list. */
>> +    spin_lock(&ctx->completion_lock);
>>       if (!req_need_defer(req, seq) && 
>> list_empty_careful(&ctx->defer_list)) {
>> +        spin_unlock(&ctx->completion_lock);
> 
> I haven't checked the sync assumptions, but it was as this since
> the very beginning, so curious if you see any hangs or other
> problems?
No, I just go over it in my mind: cq_extra and cached_cq_tail are both
updated in one completion_lock critical section, lacking of lock may
cause wrong values of cq_extra and cached_cq_tail and thus
req_need_defer() return wrong result. For example, req_need_defer() see
the updated cached_cq_tail but has the old cq_extra value. This is
possible since io_rsrc_put_work() runs in system-worker.
The result of lacking of lock is the drain request may be delayed a
little bit more or less time.
There is also a cq_extra-- in io_get_sqe(), which is the hot path, so
I incline to not touch it.
> 
> In any case, as drain is pushed to slow path, I'm all for
> simplifying synchronisation here.

  reply	other threads:[~2021-11-26  3:31 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-25  9:21 [PATCH for-5.17 0/2] small fix and code clean Hao Xu
2021-11-25  9:21 ` [PATCH 1/2] io_uring: fix no lock protection for ctx->cq_extra Hao Xu
2021-11-25 14:30   ` Pavel Begunkov
2021-11-26  3:29     ` Hao Xu [this message]
2021-11-25  9:21 ` [PATCH 2/2] io_uring: better to use REQ_F_IO_DRAIN for req->flags Hao Xu
2021-11-25 14:26   ` Pavel Begunkov
2021-11-25 16:00 ` [PATCH for-5.17 0/2] small fix and code clean Jens Axboe

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