On 2/19/20 2:10 PM, Ming Lei wrote: > On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote: >> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote: >>> For some reason, device may be in one situation which can't handle >>> FS request, so STS_RESOURCE is always returned and the FS request >>> will be added to hctx->dispatch. However passthrough request may >>> be required at that time for fixing the problem. If passthrough >>> request is added to scheduler queue, there isn't any chance for >>> blk-mq to dispatch it given we prioritize requests in hctx->dispatch. >>> Then the FS IO request may never be completed, and IO hang is caused. >>> >>> So passthrough request has to be added to hctx->dispatch directly. >>> >>> Fix this issue by inserting passthrough request into hctx->dispatch >>> directly. Then it becomes consistent with original legacy IO request >>> path, in which passthrough request is always added to q->queue_head. >> >> Do you have a description of an actual problem this fixes? Maybe even >> a reproducer for blktests? >> > > It is reported by one RH customer in the following test case: > > 1) Start IO on Emulex FC host > 2) Fail one controller, wait 5 minutes > 3) Bring controller back online > > When we trace the problem, it is found that FS request started in device_add_disk() > from scsi disk probe context stuck because scsi_queue_rq() always return > STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn(). > > The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see > BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn(). > > However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work > when the FS request can't be dispatched to LLD. And SCSI stack should > have been allowed to handle this passthrough rquest. But it can't reach SCSI stack > via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is > empty. > > The legacy IO request code always added passthrough request into head of q->queue_head > directly instead of scheduler queue or sw queue, so no such issue. > > So far not figured out one blktests test case, but the problem is real. > > BTW, I just found we need the extra following change: > > @@ -1301,7 +1301,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, > q->mq_ops->commit_rqs(hctx); > > spin_lock(&hctx->lock); > - list_splice_init(list, &hctx->dispatch); > + list_splice_tail_init(list, &hctx->dispatch); > spin_unlock(&hctx->lock); > Is it fine to add to tail as the requests on dispatch would be reordered? A, B, C and D are on the list. Suppose A is failed and the new list would become B, C D, A? Dongli Zhang
