On Wed, Jan 17 2018 at 7:54P -0500,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> But sure, I suppose there is something I missed when refactoring Ming's
> change to get it acceptable for upstream. I went over the mechanical
> nature of what I did many times (comparing Ming's v4 to my v5).
And yes there is one subtlety that I missed.
> The call to blk_mq_request_bypass_insert will only occur via
> __blk_mq_fallback_to_insert. Which as the name implies this is not the
> fast path. This will occur if the underlying blk-mq device cannot get
> resources it needs in order to issue the request. Specifically: if/when
> in __blk_mq_try_issue_directly() the hctx is stopped, or queue is
> quiesced, or it cannot get the driver tag or dispatch_budget (in the
> case of scsi-mq).
>
> The same fallback, via call to blk_mq_request_bypass_insert, occured
> with Ming's v4 though.
Turns out Ming's v4 doesn't fallback to insert for the "or it cannot get
the driver tag or dispatch_budget" case.
This patch should fix it (Laurence, please report back on if this fixes
your list_add corruption, pretty sure it will):
From: Mike Snitzer <snitzer@xxxxxxxxxx>
Date: Wed, 17 Jan 2018 22:02:07 -0500
Subject: [PATCH] blk mq: don't blk_mq_request_bypass_insert _and_ return BLK_STS_RESOURCE
It isn't ever valid to call blk_mq_request_bypass_insert() and return
BLK_STS_RESOURCE.
Unfortunately after commit 396eaf21ee ("blk-mq: improve DM's blk-mq IO
merging via blk_insert_cloned_request feedback") we do just that if
blk_mq_request_direct_issue() cannot get the resources (driver_tag or
dispatch_budget) needed to directly issue a request. This will lead to
"list_add corruption" because blk-mq submits the IO but then reports
that it didn't (BLK_STS_RESOURCE in this case).
Fix this by simply returning BLK_STS_RESOURCE for this case.
Fixes: 396eaf21ee ("blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback")
Reported-by: Laurence Oberman <loberman@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
---
block/blk-mq.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c418858a60ef..8bee37239255 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1799,20 +1799,18 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
if (q->elevator && !bypass_insert)
goto insert;
- if (!blk_mq_get_driver_tag(rq, NULL, false))
- goto insert;
-
- if (!blk_mq_get_dispatch_budget(hctx)) {
+ if (!blk_mq_get_driver_tag(rq, NULL, false) ||
+ !blk_mq_get_dispatch_budget(hctx)) {
+ /* blk_mq_put_driver_tag() is idempotent */
blk_mq_put_driver_tag(rq);
+ if (bypass_insert)
+ return BLK_STS_RESOURCE;
goto insert;
}
return __blk_mq_issue_directly(hctx, rq, cookie);
insert:
__blk_mq_fallback_to_insert(rq, run_queue, bypass_insert);
- if (bypass_insert)
- return BLK_STS_RESOURCE;
-
return BLK_STS_OK;
}
--
2.15.0
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