From: Jens Axboe <axboe@kernel.dk>
To: Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org
Subject: Re: [PATCH 3/6] io_uring: fix racy overflow count reporting
Date: Thu, 30 Jul 2020 11:41:57 -0600 [thread overview]
Message-ID: <1bd125ff-4077-fda6-1c93-04c47ebdd1cc@kernel.dk> (raw)
In-Reply-To: <07b1ee0e-72d9-a202-34ac-8095628c72f8@gmail.com>
On 7/30/20 11:33 AM, Pavel Begunkov wrote:
> On 30/07/2020 20:18, Jens Axboe wrote:
>> On 7/30/20 9:43 AM, Pavel Begunkov wrote:
>>> All ->cq_overflow modifications should be under completion_lock,
>>> otherwise it can report a wrong number to the userspace. Fix it in
>>> io_uring_cancel_files().
>>>
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> ---
>>> fs/io_uring.c | 3 +--
>>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 11f4ab87e08f..6e2322525da6 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -7847,10 +7847,9 @@ static void io_uring_cancel_files(struct io_ring_ctx *ctx,
>>> clear_bit(0, &ctx->cq_check_overflow);
>>> ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
>>> }
>>> - spin_unlock_irq(&ctx->completion_lock);
>>> -
>>> WRITE_ONCE(ctx->rings->cq_overflow,
>>> atomic_inc_return(&ctx->cached_cq_overflow));
>>> + spin_unlock_irq(&ctx->completion_lock);
>>
>> Torn writes? Not sure I see what the issue here, can you expand?
>
> No, just off-by-one(many). E.g.
>
> let: cached_overflow = 0;
>
> CPU 1 | CPU 2
> ====================================================================
> t = ++cached_overflow // t == 1 |
> | t2 = ++cached_overflow // t2 == 2
> | WRITE_ONCE(cq_overflow, t2)
> WRITE_ONCE(cq_overflow, t1) |
>
>
> So, ctx->rings->cq_overflow == 1, but ctx->cached_cq_overflow == 2.
> A minor problem and easy to fix.
Ah yes, good point.
--
Jens Axboe
next prev parent reply other threads:[~2020-07-30 17:42 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-30 15:43 [PATCH 0/6] 5.9 small cleanups/fixes Pavel Begunkov
2020-07-30 15:43 ` [PATCH 1/6] io_uring: de-unionise io_kiocb Pavel Begunkov
2020-07-30 15:43 ` [PATCH 2/6] io_uring: deduplicate __io_complete_rw() Pavel Begunkov
2020-07-30 15:43 ` [PATCH 3/6] io_uring: fix racy overflow count reporting Pavel Begunkov
2020-07-30 17:18 ` Jens Axboe
2020-07-30 17:33 ` Pavel Begunkov
2020-07-30 17:41 ` Jens Axboe [this message]
2020-07-30 15:43 ` [PATCH 4/6] io_uring: fix stalled deferred requests Pavel Begunkov
2020-07-30 15:43 ` [PATCH 5/6] io_uring: consolidate *_check_overflow accounting Pavel Begunkov
2020-07-30 15:43 ` [PATCH 6/6] io_uring: get rid of atomic FAA for cq_timeouts Pavel Begunkov
2020-07-30 18:08 ` [PATCH 0/6] 5.9 small cleanups/fixes Jens Axboe
2020-07-31 14:28 ` Pavel Begunkov
2020-07-31 15:10 ` Jens Axboe
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