From: Tejun Heo <tj@kernel.org>
Date: Wed, 18 Jun 2014 15:21:08 +0000 (-0400)
Subject: blk-mq: fix a memory ordering bug in blk_mq_queue_enter()
X-Git-Tag: cleanup-for-v3.18~34^2~15
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=531ed6261e7466907418b1a9971a5c71d7d250e4;p=pandora-kernel.git

blk-mq: fix a memory ordering bug in blk_mq_queue_enter()

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; unfortunately, it contains a subtle bug -
smp_wmb() in blk_mq_queue_enter() doesn't prevent prevent the cpu from
fetching @q->bypass_depth before incrementing @q->mq_usage_counter and
if freezing happens inbetween the caller can slip through and freezing
can be complete while there are active users.

Use smp_mb() instead so that bypass_depth and mq_usage_counter
modifications and tests are properly interlocked.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
---

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