[PATCHSET percpu/for-4.4] percpu_ref: make mode switching operations synchronize themselves

[Tejun Heo <tj@kernel.org>]

Hello,

Mostly for historical reasons, percpu_ref atomic/percpu mode switching
operations require the users to synchronize different operations
although there are valid usage scenarios where different types of
operations racing against each other makes sense.  This unusual
requirement led to a subtle race condition in blk-mq which was spotted
by Akinobu Mita[1].

Akinobu proposed a blk-mq specific fix where it adds an enclosing
mutex around the percpu_ref operations; however, this is a percpu_ref
deficiency and fixing it at the source is the better long term
approach.  Unfortunately, this ended up being a somewhat invasive set
of changes and the right thing to do likely is applying Akinobu's fix
for 4.3 / -stable, applying this patchset for 4.4 window and reverting
Akinobu's patch after the 4.4 merge window.

Akinobu, can you please verify that this patchset makes the race
condition go away?

This patchset contains the following five patches.

 0001-percpu_ref-remove-unnecessary-RCU-grace-period-for-s.patch
 0002-percpu_ref-reorganize-__percpu_ref_switch_to_atomic-.patch
 0003-percpu_ref-unify-staggered-atomic-switching-wait-beh.patch
 0004-percpu_ref-restructure-operation-mode-switching.patch
 0005-percpu_ref-allow-operation-mode-switching-operations.patch

0001-0004 are prep patches.  0001 and 0003 involve minor behavior
changes which shouldn't affect users.  0005 implements interal
synchronization among percpu mode switching operations.

diffstat follows.  Thanks.

 lib/percpu-refcount.c |  166 +++++++++++++++++++++++++++-----------------------
 1 file changed, 92 insertions(+), 74 deletions(-)

[1] http://lkml.kernel.org/g/1443287365-4244-7-git-send-email-akinobu.mita@gmail.com

--
tejun