On Tue, Apr 10, 2018 at 12:58:04PM +0000, Bart Van Assche wrote: > On Tue, 2018-04-10 at 16:41 +0800, Ming Lei wrote: > > On Mon, Apr 09, 2018 at 06:34:55PM -0700, Bart Van Assche wrote: > > > If a completion occurs after blk_mq_rq_timed_out() has reset > > > rq->aborted_gstate and the request is again in flight when the timeout > > > > Given rq->aborted_gstate is reset only for BLK_EH_RESET_TIMER, I > > think you are addressing the race between normal completion and > > the timeout of BLK_EH_RESET_TIMER. > > Yes, that's correct. I will make this more explicit. > > > > expires then a request will be completed twice: a first time by the > > > timeout handler and a second time when the regular completion occurs. > > > > This patch looks simpler, and more like the previous model of > > using blk_mark_rq_complete(). > > That's also correct. > > > I have one question: > > > > - with this patch, rq's state is updated atomically as cmpxchg. Suppose > > one rq is timed out, the rq's state is updated as MQ_RQ_COMPLETE by > > blk_mq_check_expired(), then ops->timeout() is run and > > BLK_EH_RESET_TIMER is returned, so blk_mq_add_timer(req, MQ_RQ_COMPLETE, > > MQ_RQ_IN_FLIGHT) is called to update rq's state into MQ_RQ_IN_FLIGHT. > > > > Now the original normal completion still may occur after rq's state > > becomes MQ_RQ_IN_FLIGHT, and seems there is still the double completion > > with this patch? Maybe I am wrong, please explain a bit. > > That scenario won't result in a double completion. After the timer has > been reset the block driver blk_mq_complete_request() call will change > the request state from MQ_RQ_IN_FLIGHT into MQ_RQ_COMPLETE. The next > time blk_mq_check_expired() is called it will execute the following code: > > blk_mq_change_rq_state(rq, MQ_RQ_IN_FLIGHT, MQ_RQ_COMPLETE); > > That function call only changes the request state if the current state is > IN_FLIGHT. However, the blk_mq_complete_request() call changed the request > state into COMPLETE. Hence, the above blk_mq_change_rq_state() call will > return false and the blk-mq timeout code will skip this request. If the > request would already have been reused and would have been marked again as > IN_FLIGHT then its deadline will also have been updated and the request > will be skipped by the timeout code because its deadline has not yet > expired. OK. Then I have same question with Jianchao, what is the actual double complete in linus tree between BLK_EH_RESET_TIMER and normal completion? Follows my understanding: 1) when timeout is detected on one request, its aborted_gstate is updated in blk_mq_check_expired(). 2) run synchronize_rcu(), and make sure all pending completion is done 3) run blk_mq_rq_timed_out() - ret = ops->timeout - blk_mq_rq_update_aborted_gstate(req, 0) - blk_add_timer(req); If normal completion is done between 1) and reset aborted_gstate in 3), blk_mq_complete_request() will be called, and found that aborted_gstate is set, then the rq won't be completed really. If normal completion is done after reset aborted_gstate in 3), it should be same with applying this patch. > > > And synchronize_rcu() is needed by Tejun's approach between marking > > COMPLETE and handling this rq's timeout, and the time can be quite long, > > I guess it might be easier to trigger? > > I have done what I could to trigger races between the regular completion > path and the timeout code in my tests. Without this patch if I run the > srp-test software I see crashes being reported in the rdma_rxe driver but > with this patch applied I don't see any crashes with my tests. I believe this patch may fix this issue, but I think the idea behind should be understood. Thanks, Ming
