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blk-mq: Fix stall due to recursive flush plug 

We have seen rare IO stalls as follows:

* blk_mq_plug_issue_direct() is entered with an mq_list containing two
requests.
* For the first request, it sets last == false and enters the driver's
queue_rq callback.
* The driver queue_rq callback indirectly calls schedule() which calls
blk_flush_plug(). This may happen if the driver has the
BLK_MQ_F_BLOCKING flag set and is allowed to sleep in ->queue_rq.
* blk_flush_plug() handles the remaining request in the mq_list. mq_list
is now empty.
* The original call to queue_rq resumes (with last == false).
* The loop in blk_mq_plug_issue_direct() terminates because there are no
remaining requests in mq_list.

The IO is now stalled because the last request submitted to the driver
had last == false and there was no subsequent call to commit_rqs().

Fix this by returning early in blk_mq_flush_plug_list() if rq_count is 0
which it will be in the recursive case, rather than checking if the
mq_list is empty. At the same time, adjust one of the callers to skip
the mq_list empty check as it is not necessary.

Fixes: dc5fc361d8 ("block: attempt direct issue of plug list")
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230714101106.3635611-1-ross.lagerwall@citrix.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Ross Lagerwall 2023-07-14 11:11:06 +01:00 committed by Jens Axboe
parent 9f87fc4d72
commit 7090426351
2 changed files with 9 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
block/blk-core.c
View File

@ -1144,8 +1144,7 @@ void __blk_flush_plug(struct blk_plug *plug, bool from_schedule)
{
if (!list_empty(&plug->cb_list))
flush_plug_callbacks(plug, from_schedule);
if (!rq_list_empty(plug->mq_list))
blk_mq_flush_plug_list(plug, from_schedule);
blk_mq_flush_plug_list(plug, from_schedule);
/*
* Unconditionally flush out cached requests, even if the unplug
* event came from schedule. Since we know hold references to the

9
block/blk-mq.c
View File

@ -2754,7 +2754,14 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
{
struct request *rq;
if (rq_list_empty(plug->mq_list))
/*
* We may have been called recursively midway through handling
* plug->mq_list via a schedule() in the driver's queue_rq() callback.
* To avoid mq_list changing under our feet, clear rq_count early and
* bail out specifically if rq_count is 0 rather than checking
* whether the mq_list is empty.
*/
if (plug->rq_count == 0)
return;
plug->rq_count = 0;