[PATCHSET block/for-next] blk-mq: update freezing

[Tejun Heo <tj@kernel.org>]

Hello, Jens.

While reading through blk-mq, I spotted several issues and this
patchset addresses them.  The biggest one is how freezing is
implemented.  Coupling it with bypassing doesn't seem to work well and
there's a subtle bug in the perpcu switch implementation.

I don't think open-coding this level of percpu logic is a good idea.
It tends to be very error-prone and difficult to follow.  The barrier
problem is the only thing I spotted but it's very difficult to say
that it's correct.  percpu_ref already implements most of what's
necessary to implement this sort of percpu switch and I added the
missing bits in a recent patchset and converted blk-mq freezing
mechanism to use it in this patch.

It's far simpler and easier to wrap one's head around, and, as it's
shared with other mechanisms including aio and cgroups, we should have
better testing coverage and more eyes scrutinizing it.

This patchset contains the following six patches.

 0001-blk-mq-make-blk_mq_stop_hw_queue-reliably-block-queu.patch
 0002-blk-mq-fix-a-memory-ordering-bug-in-blk_mq_queue_ent.patch
 0003-block-blk-mq-draining-can-t-be-skipped-even-if-bypas.patch
 0004-blk-mq-decouble-blk-mq-freezing-from-generic-bypassi.patch
 0005-blk-mq-collapse-__blk_mq_drain_queue-into-blk_mq_fre.patch
 0006-blk-mq-use-percpu_ref-for-mq-usage-count.patch

0001-0003 are fix patches that can be applied separately.

0004 decouples blk-mq freezing from queue bypassing.

0005-0006 replace the custom percpu switch with percpu_ref.

This patchset is on top of

 percpu/for-3.17 6fbc07bbe2b5 ("percpu: invoke __verify_pcpu_ptr() from the generic part of accessors and operations")
+[1] [PATCHSET percpu/for-3.17] percpu: implement percpu_ref_reinit()

and available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git review-mq-percpu_ref

 block/blk-core.c       |   13 ++++---
 block/blk-mq.c         |   90 ++++++++++++++++---------------------------------
 block/blk-mq.h         |    2 -
 block/blk-sysfs.c      |    2 -
 include/linux/blkdev.h |    4 +-
 5 files changed, 44 insertions(+), 67 deletions(-)

Thanks.

--
tejun

[1] http://lkml.kernel.org/g/1403053685-28240-1-git-send-email-tj@kernel.org