From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Bart Van Assche To: "ming.lei@redhat.com" CC: "linux-scsi@vger.kernel.org" , "hch@infradead.org" , "linux-block@vger.kernel.org" , "axboe@fb.com" , "jejb@linux.vnet.ibm.com" , "martin.petersen@oracle.com" Subject: Re: [PATCH 05/14] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed Date: Tue, 1 Aug 2017 16:14:07 +0000 Message-ID: <1501604045.2475.8.camel@wdc.com> References: <20170731165111.11536-1-ming.lei@redhat.com> <20170731165111.11536-7-ming.lei@redhat.com> <1501544540.2466.31.camel@wdc.com> <20170801104433.GC31452@ming.t460p> In-Reply-To: <20170801104433.GC31452@ming.t460p> Content-Type: text/plain; charset="iso-8859-1" MIME-Version: 1.0 Sender: linux-scsi-owner@vger.kernel.org List-ID: 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 concurr= ently > > and since clearing and testing happens without any locks held I'm afrai= d this > > patch introduces the following race conditions: > > [ ... ] > > * Checking BLK_MQ_S_BUSY after requests have been removed from the disp= atch 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. >=20 > 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. Thanks, Bart.=