On Fri, 2017-04-28 at 07:42 -0600, Jens Axboe wrote: > On 04/28/2017 01:32 AM, Ming Lei wrote: > > We have freezed queue already, not necessary to call > > blk_mq_quiesce_queue() any more, so remove it. > > Are you sure? It ensures that we also aren't in the middle of > blk_mq_make_request(), we need a stable view of the sched > status throughout that. Hello Jens, My understanding is that blk_mq_freeze_queue() provides stronger guarantees than blk_mq_quiesce_queue(). The former waits until all pending requests have finished while the latter only waits until pending .queue_rq() calls have finished. blk_mq_freeze_queue() also causes new blk_get_request() calls to wait until blk_mq_unfreeze_queue() is called while blk_get_request() can still succeed after blk_mq_quiesce_queue() returned and before blk_mq_start_stopped_hw_queues() is called. Regarding blk_mq_make_request(): I think that the blk_queue_enter() call in generic_make_request() prevents that blk_mq_make_request() gets called after a queue has been frozen. Bart.
