Messages in this thread | | | Subject | Re: [PATCH] blk-mq: Put driver tag in blk_mq_dispatch_rq_list() when no budget | From | John Garry <> | Date | Mon, 20 Apr 2020 09:22:58 +0100 |
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On 18/04/2020 03:43, Bart Van Assche wrote: > On 2020-04-16 04:18, John Garry wrote: >> If in blk_mq_dispatch_rq_list() we find no budget, then we break of the >> dispatch loop, but the request may keep the driver tag, evaulated >> in 'nxt' in the previous loop iteration. >> >> Fix by putting the driver tag for that request. >> >> Signed-off-by: John Garry <john.garry@huawei.com> >> >> diff --git a/block/blk-mq.c b/block/blk-mq.c >> index 8e56884fd2e9..a7785df2c944 100644 >> --- a/block/blk-mq.c >> +++ b/block/blk-mq.c >> @@ -1222,8 +1222,10 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, >> rq = list_first_entry(list, struct request, queuelist); >> >> hctx = rq->mq_hctx; >> - if (!got_budget && !blk_mq_get_dispatch_budget(hctx)) >> + if (!got_budget && !blk_mq_get_dispatch_budget(hctx)) { >> + blk_mq_put_driver_tag(rq); >> break; >> + } >> >> if (!blk_mq_get_driver_tag(rq)) { >> /* > > Is this something that can only happen if q->mq_ops->queue_rq(hctx, &bd) > returns another value than BLK_STS_OK, BLK_STS_RESOURCE and > BLK_STS_DEV_RESOURCE?
Right, as that case is handled in blk_mq_handle_dev_resource()
If so, please add a comment in the source code > that explains this.
So important that we should now do this in an extra patch?
> > Is this perhaps a bug fix for 0bca799b9280 ("blk-mq: order getting > budget and driver tag")? If so, please mention this and add Cc tags for > the people who were Cc-ed on that patch.
So it looks like 0bca799b9280 had a flaw, but I am not sure if anything got broken there and worthy of stable backport.
I found this issue while debugging Ming's blk-mq cpu hotplug patchset, which I feel is ready to merge.
Having said that, this nasty issue did take > 1 day for me to debug... so let me know.
Thanks, John
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