Re: [PATCH 3/6] io_uring: fix racy overflow count reporting

[Jens Axboe <axboe@kernel.dk>]

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