From: Tejun Heo <tj@kernel.org>
Date: Fri, 13 Apr 2012 21:50:53 +0000 (-0700)
Subject: blkcg: make sure blkg_lookup() returns %NULL if @q is bypassing
X-Git-Tag: v3.5-rc1~42^2~18
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=80fd99792b0b9f162abdf3da12fb10eb9eb5f321;p=pandora-kernel.git

blkcg: make sure blkg_lookup() returns %NULL if @q is bypassing

Currently, blkg_lookup() doesn't check @q bypass state.  This patch
updates blk_queue_bypass_start() to do synchronize_rcu() before
returning and updates blkg_lookup() to check blk_queue_bypass() and
return %NULL if bypassing.  This ensures blkg_lookup() returns %NULL
if @q is bypassing.

This is to guarantee that nobody is accessing policy data while @q is
bypassing, which is necessary to allow replacing blkio_cgroup->pd[] in
place on policy [de]activation.

v2: Added more comments explaining bypass guarantees as suggested by
    Vivek.

v3: Added more comments explaining why there's no synchronize_rcu() in
    blk_cleanup_queue() as suggested by Vivek.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---

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