On Wed, May 31, 2017 at 03:37:30PM +0000, Bart Van Assche wrote: > On Wed, 2017-05-31 at 20:37 +0800, Ming Lei wrote: > > > > + /* wait until queue is unquiesced */ > > + wait_event_cmd(q->quiesce_wq, !blk_queue_quiesced(q), > > + may_sleep ? > > + srcu_read_unlock(&hctx->queue_rq_srcu, *srcu_idx) : > > + rcu_read_unlock(), > > + may_sleep ? > > + *srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu) : > > + rcu_read_lock()); > > + > > if (q->elevator) > > goto insert; > > What I see is that in this patch a new waitqueue has been introduced > (quiesce_wq) and also that an explanation of why you think this new waitqueue > is needed is missing completely. Why is it that you think that the > synchronize_scru() and synchronize_rcu() calls in blk_mq_quiesce_queue() are > not sufficient? If this new waitqueue is not needed, please remove that > waitqueue again. OK, the reason is simple, and it is only related with direct issue. Under this situation, when the queue is quiesced, we have to - insert the current request into sw queue(scheduler queue) OR -wait until queue becomes unquiesced like what this patch is doing The disadvantage of the 1st way is that we have to consider to run queue again in blk_mq_unquiesce_queue() for the queued requests during quiescing. For the 2nd way(what this patch is doing), one benefit is that application can avoid to submit I/O to a quiesced queue. Another benefit is that we needn't to consider to run queue in blk_mq_unquiesce_queue(). But with cost of one waitqueue, the cost should be cheap, but if you persist on the 1st approach, I am fine to change to that. Thanks, Ming
