On Fri, May 16, 2014 at 10:57 PM, Jens Axboe <[email protected]> wrote: > 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.
Yes. But the flag can avoid to call blk_mq_start_stopped_hw_queues() unnecessarily, which needn't at most of times. Considered that the interrupt may happen with very high frequency, I suggest to introduce the extra flag. Thanks, -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
