On 2014-05-16 08:53, Jens Axboe wrote:
> On 2014-05-15 06:33, Ming Lei wrote:
>> When there isn't enough vring descriptor for adding to vq,
>> blk-mq will be put as stopped state until some of pending
>> descriptors are completed & freed.
>>
>> Unfortunately, the vq's interrupt may come just before
>> blk-mq's BLK_MQ_S_STOPPED flag is set, so the blk-mq will
>> still be kept as stopped even though lots of descriptors
>> are completed and freed in the interrupt handler. The worst
>> case is that all pending descriptors are freed in the
>> interrupt handler, and the queue is kept as stopped forever.
>>
>> This patch fixes the problem by starting/stopping blk-mq
>> with holding vq_lock.
>
> Why not just use blk_mq_start_hw_queues()?

Or, if you want to maintain current heuristics, just move the start and 
stop under the vq_lock. That should prevent the race, as far as I can 
tell. Not sure what that extra queue_stopped would buy you, seems a lot 
cleaner to just maintain this state exclusively in the queue.

-- 
Jens Axboe