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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 <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Shaohua Li 2013-12-31 11:38:50 +08:00 • committed by Jens Axboe
parent d835502f3d
commit f0276924fa
3 changed files with 38 additions and 19 deletions
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3
include/linux/blk-mq.h
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@ -36,12 +36,15 @@ struct blk_mq_hw_ctx {
struct list_head page_list;
struct blk_mq_tags *tags;
atomic_t pending_flush;
unsigned long queued;
unsigned long run;
#define BLK_MQ_MAX_DISPATCH_ORDER 10
unsigned long dispatched[BLK_MQ_MAX_DISPATCH_ORDER];
unsigned int queue_depth;
unsigned int reserved_tags;
unsigned int numa_node;
unsigned int cmd_size; /* per-request extra data */