On Sat, May 27, 2017 at 09:32:02PM +0000, Bart Van Assche wrote: > On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote: > > There are some issues in current blk_mq_quiesce_queue(): > > > > - in case of direct issue or BLK_MQ_S_START_ON_RUN, dispatch won't > > be prevented after blk_mq_quiesce_queue() is returned. > > - in theory, new RCU read-side critical sections may begin while > > synchronize_rcu() was waiting, and end after returning from > > synchronize_rcu(), then dispatch still may be run after > > synchronize_rcu() returns > > Hello Ming, Hello Bart, > > I disagree with the second part of your statement. blk_mq_quiesce_queue() You can find the similar description from line 158 to line 180. of Documentation/RCU/whatisRCU.txt. For example, there may be schedule preempt or irq handling between checking stopped flag(false) and the RCU read-side critical sections, and synchronize_*rcu() may return before running this RCU read-side critical sections. That is why we have to move the check into RCU read-side critical sections. > stops all hardware queues before calling synchronize_*rcu(). > blk_mq_sched_dispatch_requests() is called with an RCU lock held and checks > whether a hctx has been stopped before calling .queue_rq(). That is > sufficient to prevent blk_mq_try_issue_directly() to trigger a .queue_rq() > call. Our case should be very similar with example "6. ANALOGY WITH READER-WRITER LOCKING" in Documentation/RCU/whatisRCU.txt. Thanks, Ming
