Re: [PATCH] block: pop cached rq before potentially blocking rq_qos_throttle()

[Jens Axboe <axboe@kernel.dk>]

On 6/23/22 2:56 AM, Shinichiro Kawasaki wrote:
> On Jun 21, 2022 / 10:07, Jens Axboe wrote:
>> If rq_qos_throttle() ends up blocking, then we will have invalidated and
>> flushed our current plug. Since blk_mq_get_cached_request() hasn't
>> popped the cached request off the plug list just yet, we end holding a
>> pointer to a request that is no longer valid. This insta-crashes with
>> rq->mq_hctx being NULL in the validity checks just after.
>>
>> Pop the request off the cached list before doing rq_qos_throttle() to
>> avoid using a potentially stale request.
>>
>> Fixes: 0a5aa8d161d1 ("block: fix blk_mq_attempt_bio_merge and rq_qos_throttle protection")
>> Reported-by: Dylan Yudaken <dylany@fb.com>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> Thank you for the fix and sorry for the trouble. The patch passed my test set:
> 
> Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> 
> 
> BTW, may I know the workload or script which triggers the failure? I'm
> interested in if we can add a blktests test case to exercises qos throttle
> and prevent similar bugs in the future.

Not sure if there are others, but specifically blk-iocost will do an
explicit schedule() for some conditions. See the bottom of
ioc_rqos_throttle().

-- 
Jens Axboe