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Message-ID: <20230819031206.2744005-1-chengming.zhou@linux.dev>
Date: Sat, 19 Aug 2023 11:12:06 +0800
From: chengming.zhou@...ux.dev
To: axboe@...nel.dk, hch@....de, bvanassche@....org,
ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
zhouchengming@...edance.com, chuck.lever@...cle.com, lkp@...el.com,
kernel test robot <oliver.sang@...el.com>
Subject: [PATCH] blk-mq: fix mismatch between IO scheduler insert and finish
From: Chengming Zhou <zhouchengming@...edance.com>
IO scheduler has requirement that one request which has been inserted
must call finish_request() only once.
Now we have three special cases to consider:
1. rq has not insert, has complete: e.g. empty preflush
2. rq has insert, has not complete: e.g. merged requests will be freed
3. rq has insert, has twice complete: e.g. postflushes
Note case 1 which existed before, has been no problem since all the
schedulers will check in their finish_request() if the rq has been
inserted or not, like checking "rq->elv.priv[0]".
Then case 2 and case 3 are the introduced regression, we moved the
scheduler finish_request() from free phase to complete phase to solve
a deadlock problem. But it caused no finish_request() for request in
case 2, and double finish_request() for request in case 3.
So we still need finish_request() in blk_mq_free_request() to cover
case 2. And clear RQF_USE_SCHED flag to avoid double finish_request().
It should be fine since we're freeing the request now anyway.
Of course, we can also make all schedulers' finish_request() to clear
"rq->elv.priv[0]" to avoid double finish. Or clear it in blk-mq, make
the rq like not inserted as case 1.
FYI it's easy to reproduce warning in mq-deadline using this:
```
DEV=sdb
echo mq-deadline > /sys/block/$DEV/queue/scheduler
mkfs.ext4 /dev/$DEV
mount /dev/$DEV /mnt
cd /mnt
stress-ng --symlink 4 --timeout 60
echo none > /sys/block/$DEV/queue/scheduler
```
Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202308172100.8ce4b853-oliver.sang@intel.com
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
block/blk-mq.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index a6d59320e034..953f08354c8c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -685,8 +685,15 @@ static void blk_mq_finish_request(struct request *rq)
{
struct request_queue *q = rq->q;
- if (rq->rq_flags & RQF_USE_SCHED)
+ if (rq->rq_flags & RQF_USE_SCHED) {
q->elevator->type->ops.finish_request(rq);
+ /*
+ * For postflush request that may need to be
+ * completed twice, we should clear this flag
+ * to avoid double finish_request() on the rq.
+ */
+ rq->rq_flags &= ~RQF_USE_SCHED;
+ }
}
static void __blk_mq_free_request(struct request *rq)
@@ -715,6 +722,8 @@ void blk_mq_free_request(struct request *rq)
{
struct request_queue *q = rq->q;
+ blk_mq_finish_request(rq);
+
if (unlikely(laptop_mode && !blk_rq_is_passthrough(rq)))
laptop_io_completion(q->disk->bdi);
--
2.41.0
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