On Tue, Aug 01, 2017 at 04:14:07PM +0000, Bart Van Assche wrote: > On Tue, 2017-08-01 at 18:44 +0800, Ming Lei wrote: > > On Mon, Jul 31, 2017 at 11:42:21PM +0000, Bart Van Assche wrote: > > > Since setting, clearing and testing of BLK_MQ_S_BUSY can happen concurrently > > > and since clearing and testing happens without any locks held I'm afraid this > > > patch introduces the following race conditions: > > > [ ... ] > > > * Checking BLK_MQ_S_BUSY after requests have been removed from the dispatch list > > > but before that bit is cleared, resulting in test_bit(BLK_MQ_S_BUSY, &hctx->state) > > > reporting that the BLK_MQ_S_BUSY > > > has been set although there are no requests > > > on the dispatch list. > > > > That won't be a problem, because dispatch will be started in the > > context in which dispatch list is flushed, since the BUSY bit > > is cleared after blk_mq_dispatch_rq_list() returns. So no I/O > > hang. > > Hello Ming, > > Please consider changing the name of the BLK_MQ_S_BUSY constant. That bit > is used to serialize dispatching requests from the hctx dispatch list but > that's not clear from the name of that constant. Actually what we want to do is to stop taking request from sw/scheduler queue when ->dispatch aren't flushed completely, I think BUSY isn't a bad name for this case, or how about DISPATCH_BUSY? or FLUSHING_DISPATCH? After thinking about the handling further, we can set the BUSY bit just when adding requests to ->dispatch, and clear the bit after returning from blk_mq_dispatch_rq_list() when the current local list(from ->dispatch) is flushed completely and ->dispatch is empty. This way can minimize the race window, and still safe, because we always move on to dispatch either new request is added to ->dispatch or ->dispatch is flushed completely. But anyway comment should be added for clarifying the fact. -- Ming
