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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

-- 
Pavel Begunkov