Re: [PATCH 3/3] blk-mq: Fix the queue freezing mechanism

From: Tejun Heo
Date: Thu Sep 24 2015 - 12:54:04 EST

On Thu, Sep 24, 2015 at 09:43:48AM -0700, Bart Van Assche wrote:
> On 09/23/2015 08:23 PM, Ming Lei wrote:
> >IMO, mq_freeze_depth should only be accessed in slow path, and looks
> >the race just happens during the small window between increasing
> >'mq_freeze_depth' and killing the percpu counter.
> Hello Ming,
> My concern is that *not* checking mq_freeze_depth in the hot path can cause
> a livelock. If there is a software layer, e.g. multipathd, that periodically
> submits new commands and if these commands take time to process e.g. because
> the transport layer is unavailable, how to guarantee that freezing ever
> succeeds without checking mq_freeze_depth in the hot path ?

I couldn't tell what the patch was trying to do from the patch
description, so including the above prolly is a good idea. Isn't the
above guaranteed by percpu_ref_kill() preventing new tryget_live()'s?
Also, what does the barriers do in your patch?

The only race condition that I can see there is if unfreeze and freeze
race each other and freeze tries to kill the ref which hasn't finished
reinit yet. We prolly want to put mutexes around freeze/unfreeze so
that they're serialized if something like that can happen (it isn't a
hot path to begin with).


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