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blk-mq: Don't reserve a tag for flush request

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Reserving a tag (request) for flush to avoid dead lock is a overkill. A
tag is valuable resource. We can track the number of flush requests and
disallow having too many pending flush requests allocated. With this
patch, blk_mq_alloc_request_pinned() could do a busy nop (but not a dead
loop) if too many pending requests are allocated and new flush request
is allocated. But this should not be a problem, too many pending flush
requests are very rare case.

I verified this can fix the deadlock caused by too many pending flush
requests.

Signed-off-by: Shaohua Li <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Shaohua Li 2013-12-31 11:38:50 +08:00 • committed by Jens Axboe
commit f0276924fa
3 changed files with 38 additions and 19 deletions
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8
block/blk-flush.c
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@ -284,9 +284,8 @@ static void mq_flush_work(struct work_struct *work)
q = container_of(work, struct request_queue, mq_flush_work);
/* We don't need set REQ_FLUSH_SEQ, it's for consistency */
rq = blk_mq_alloc_request(q, WRITE_FLUSH|REQ_FLUSH_SEQ,
__GFP_WAIT|GFP_ATOMIC, true);
__GFP_WAIT|GFP_ATOMIC, false);
rq->cmd_type = REQ_TYPE_FS;
rq->end_io = flush_end_io;
@ -408,8 +407,11 @@ void blk_insert_flush(struct request *rq)
/*
* @policy now records what operations need to be done. Adjust
* REQ_FLUSH and FUA for the driver.
* We keep REQ_FLUSH for mq to track flush requests. For !FUA,
* we never dispatch the request directly.
*/
rq->cmd_flags &= ~REQ_FLUSH;
if (rq->cmd_flags & REQ_FUA)
rq->cmd_flags &= ~REQ_FLUSH;
if (!(fflags & REQ_FUA))
rq->cmd_flags &= ~REQ_FUA;