On Sun, 2017-05-28 at 19:11 +0800, Ming Lei wrote: > 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 > > > > 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. Hello Ming, Have you noticed that in the current blk-mq code the test of the hctx stopped flag already occurs *inside* an RCU read-side critical section? __blk_mq_run_hw_queue() grabs an RCU read lock before blk_mq_sched_dispatch_requests() calls blk_mq_hctx_stopped(). Bart.
