Re: Race in block/blk-mq-sched.c blk_mq_sched_dispatch_requests

[Jens Axboe <axboe@kernel.dk>]

On 1/17/24 1:22 PM, Bart Van Assche wrote:
> On 1/17/24 12:17, Jens Axboe wrote:
>> On 1/17/24 1:16 PM, Gabriel Ryan wrote:
>>> We found a race in the block message queue for kernel v5.18-rc5 using
>>> a race testing tool we are developing. We are reporting this race
>>> because it appears to be potentially harmful. The race occurs in
>>>
>>> block/blk-mq-sched.c:333 blk_mq_sched_dispatch_requests
>>>
>>>      hctx->run++;
>>>
>>> where multiple threads can schedule dispatch requests and increment
>>> the request counter htctx->run simultaneously. This appears to lead to
>>> undefined behavior where multiple conflicting updates to the hctx->run
>>> value could result in it not matching the number of requests that
>>> have been scheduled with calls to blk_mq_sched_dispatch_requests.
>>
>> I suggest you take a closer look at how that variable is actually
>> used.
> 
> It's probably a good idea to explain this in a comment above the
> code that increments hctx->runs because others may also be wondering
> what the impact is of concurrent hctx->runs increments.

If you do a quick grep, you'll very quickly see that it's just used
for debugfs output. Being racy is not a problem. It should just get
removed, honestly, like I did with some of the other accounting some
time ago.

-- 
Jens Axboe