On Wed, Mar 15, 2017 at 09:34:50PM +0000, Bart Van Assche wrote: > On Wed, 2017-03-15 at 20:18 +0800, Ming Lei wrote: > > On Wed, Mar 15, 2017 at 12:07:37AM +0000, Bart Van Assche wrote: > > > Both the old and the new check look racy to me. The REQ_ATOM_STARTED bit can > > > be changed concurrently by blk_mq_start_request(), __blk_mq_finish_request() > > > > blk_mq_start_request() and __blk_mq_finish_request() won't be run concurrently. > > > > From view of __blk_mq_finish_request(): > > > > - if it is run from merge queue io path(blk_mq_merge_queue_io()), > > blk_mq_start_request() can't be run at all, and the COMPLETE flag > > is kept as previous value(zero) > > > > - if it is run from normal complete path, COMPLETE flag is cleared > > before the req/tag is released to tag set. > > > > so there isn't race in blk_mq_start_request() vs. __blk_mq_finish_request() > > wrt. timeout. > > > > > or __blk_mq_requeue_request(). Another issue with this function is that the > > > > __blk_mq_requeue_request() can be run from two pathes: > > > > - dispatch failure, in which case the req/tag isn't released to tag set > > > > - IO completion path, in which COMPLETE flag is cleared before requeue. > > > > so I can't see races with timeout in case of start rq vs. requeue rq. > > > > > request passed to this function can be reinitialized concurrently. > > Hello Ming, > > You misinterpret what I wrote. I was referring to manipulation of > REQ_ATOM_STARTED from different contexts and not to what you explained. This patch addresses one race between timeout and pre-queuing I/O in block layer (before .queue_rq), please focus on this patch. And that is definitely correct. Also I am happy to discuss this kind of races, but maybe we can do that in another thread if you can describe the issue clearly. -- Ming