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blk-mq: use percpu_ref for mq usage count

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Currently, blk-mq uses a percpu_counter to keep track of how many
usages are in flight.  The percpu_counter is drained while freezing to
ensure that no usage is left in-flight after freezing is complete.
blk_mq_queue_enter/exit() and blk_mq_[un]freeze_queue() implement this
per-cpu gating mechanism.

This type of code has relatively high chance of subtle bugs which are
extremely difficult to trigger and it's way too hairy to be open coded
in blk-mq.  percpu_ref can serve the same purpose after the recent
changes.  This patch replaces the open-coded per-cpu usage counting
and draining mechanism with percpu_ref.

blk_mq_queue_enter() performs tryget_live on the ref and exit()
performs put.  blk_mq_freeze_queue() kills the ref and waits until the
reference count reaches zero.  blk_mq_unfreeze_queue() revives the ref
and wakes up the waiters.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Tejun Heo 2014-07-01 10:34:38 -06:00 • committed by Jens Axboe
parent 72d6f02a8d
commit add703fda9
2 changed files with 31 additions and 40 deletions
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3
include/linux/blkdev.h
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@ -21,6 +21,7 @@
#include <linux/bsg.h>
#include <linux/smp.h>
#include <linux/rcupdate.h>
#include <linux/percpu-refcount.h>
#include <asm/scatterlist.h>
@ -484,7 +485,7 @@ struct request_queue {
#endif
struct rcu_head rcu_head;
wait_queue_head_t mq_freeze_wq;
struct percpu_counter mq_usage_counter;
struct percpu_ref mq_usage_counter;
struct list_head all_q_node;
struct blk_mq_tag_set *tag_set;