Messages in this thread | | | From | Roman Penyaev <> | Date | Wed, 10 Aug 2016 10:42:09 +0200 | Subject | Re: [PATCH v2 1/1] blk-mq: fix hang caused by freeze/unfreeze sequence |
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Hi,
On Wed, Aug 10, 2016 at 5:55 AM, Tejun Heo <tj@kernel.org> wrote: > Hello, > > On Mon, Aug 08, 2016 at 01:39:08PM +0200, Roman Pen wrote: >> Long time ago there was a similar fix proposed by Akinobu Mita[1], >> but it seems that time everyone decided to fix this subtle race in >> percpu-refcount and Tejun Heo[2] did an attempt (as I can see that >> patchset was not applied). > > So, I probably forgot about it while waiting for confirmation of fix. > Can you please verify that the patchset fixes the issue? I can apply > the patchset right away.
I have not checked your patchset but according to my understanding it should not fix *this* issue. What happens here is a wrong order of invocation of percpu_ref_reinit() and percpu_ref_kill(). So what was observed is the following:
CPU#0 CPU#1 ---------------- ----------------- percpu_ref_kill()
percpu_ref_kill() << atomic reference does percpu_ref_reinit() << not guarantee the order
blk_mq_freeze_queue_wait() !! HANG HERE
percpu_ref_reinit()
blk_mq_freeze_queue_wait() on CPU#1 expects percpu-refcount to be switched to ATOMIC mode (killed), but that does not happen, because CPU#2 was faster and has been switched percpu-refcount to PERCPU mode.
This race happens inside blk-mq, because invocation of kill/reinit is controlled by the reference counter, which does not guarantee the order of the following functions calls (kill/reinit).
So the fix is the same as originally proposed by Akinobu Mita, but the issue is different.
But of course I can run tests on top of your series, just to verify that everything goes smoothly and internally percpu-refcount members are consistent.
-- Roman
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