On Fri, Jun 19, 2020 at 12:06:57PM -0400, Mike Snitzer wrote: > On Fri, Jun 19 2020 at 6:11am -0400, > Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > Hi Mike, > > > > On Fri, Jun 19, 2020 at 05:42:50AM -0400, Mike Snitzer wrote: > > > Hi Ming, > > > > > > Thanks for the patch! But I'm having a hard time understanding what > > > you've written in the patch header, > > > > > > On Fri, Jun 19 2020 at 4:42am -0400, > > > Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > > > > > dm-rq won't stop queue, meantime blk-mq won't stop one queue too, so > > > > remove the check. > > > > > > It'd be helpful if you could unpack this with more detail before going on > > > to explain why using blk_queue_quiesced, despite dm-rq using > > > blk_mq_queue_stopped, would also be ineffective. > > > > > > SO: > > > > > > > dm-rq won't stop queue > > > > > > 1) why won't dm-rq stop the queue? Do you mean it won't reliably > > > _always_ stop the queue because of the blk_mq_queue_stopped() check? > > > > device mapper doesn't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues. > > > > > > > > > meantime blk-mq won't stop one queue too, so remove the check. > > > > > > 2) Meaning?: blk_mq_queue_stopped() will return true even if only one hw > > > queue is stopped, given blk-mq must stop all hw queues a positive return > > > from this blk_mq_queue_stopped() check is incorrectly assuming it meanss > > > all hw queues are stopped. > > > > blk-mq won't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues for > > dm-rq's queue too, so dm-rq's hw queue won't be stopped. > > > > BTW blk_mq_stop_hw_queue or blk_mq_stop_hw_queues are supposed to be > > used for throttling queue. > > I'm going to look at actually stopping the queue (using one of these > interfaces). I didn't realize I wasn't actually stopping the queue. > The intent was to do so. > > In speaking with Jens yesterday about freeze vs stop: it is clear that > dm-rq needs to still be able to allocate new requests, but _not_ call > the queue_rq to issue the requests, while "stopped" (due to dm-mpath > potentially deferring retries of failed requests because of path failure > while quiescing the queue during DM device suspend). But that freezing > the queue goes too far because it won't allow such request allocation. Freezing shouldn't be a good choice for driver usually, and quiesce is exactly what you expect: request allocation is allowed, meantime, no .queue_rq is possible. > > > > > dm_stop_queue() actually tries to quiesce hw queues via blk_mq_quiesce_queue(), > > > > we can't check via blk_queue_quiesced for avoiding unnecessary queue > > > > quiesce because the flag is set before synchronize_rcu() and dm_stop_queue > > > > may be called when synchronize_rcu from another blk_mq_quiesce_queue is > > > > in-progress. > > > > > > But I'm left with questions/confusion on this too: > > > > > > 1) you mention blk_queue_quiesced instead of blk_mq_queue_stopped, so I > > > assume you mean that: not only is blk_mq_queue_stopped() > > > ineffective, blk_queue_quiesced() would be too? > > > > blk_mq_queue_stopped isn't necessary because dm-rq's hw queue won't be > > stopped by anyone, meantime replacing it with blk_queue_quiesced() is wrong. > > > > > > > > 2) the race you detail (with competing blk_mq_quiesce_queue) relative to > > > synchronize_rcu() and testing "the flag" is very detailed yet vague. > > > > If two code paths are calling dm_stop_queue() at the same time, one path may > > return immediately and it is wrong, sine synchronize_rcu() from another path > > may not be done. > > > > > > > > Anyway, once we get this heaader cleaned up a bit more I'll be happy to > > > get this staged as a stable@ fix for 5.8 inclusion ASAP. > > > > This patch isn't a fix, and it shouldn't be related with rhel8's issue. > > I realize that now. I've changed the patch header to be a bit clearer > and staged it for 5.9, see: > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.9&id=06e788ed59e0095b679bdce9e39c1a251032ae62 Thanks! -- Ming
