linux-kernel - Re: [PATCH 7/8] Clean up workqueue.c with respect to the freezer based cpu-hotplug

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Message-ID: <20070412022220.GA10176@in.ibm.com>
Date:	Thu, 12 Apr 2007 07:52:20 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Gautham R Shenoy <ego@...ibm.com>, akpm@...ux-foundation.org,
	paulmck@...ibm.com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	mingo@...e.hu, dipankar@...ibm.com, dino@...ibm.com,
	masami.hiramatsu.pt@...achi.com
Subject: Re: [PATCH 7/8] Clean up workqueue.c with respect to the freezer based cpu-hotplug

On Tue, Apr 03, 2007 at 10:48:20PM +0530, Srivatsa Vaddagiri wrote:
> > Actually, we should do this before destroy_workqueue() calls flush_workqueue().
> > Otherwise flush_cpu_workqueue() can hang forever in a similar manner.
> 
> Yep. I guess these are a class of freezer deadlocks very similar to vfork
> parent waiting on child case. I get a feeling these should become common
> outside of kthread too (A waits on B for something, B gets frozen, which
> means A won't freeze causing freezer to fail). Can freezer detect this
> dependency somehow and thaw B automatically? Probably not that easy ..

I wonder if there is some value in "enforcing" an order in which
processes get frozen i.e freeze A first before B. That may solve the
deadlocks we have been discussing wrt kthread_stop and flush_workqueue
as well.

The idea is similar to how deadlock wrt multiple locks are solved -
where a ordering is enforced. Take Lock A first before Lock B. 

If process A waits on B (like in kthread_stop or flush_workqueue), then if we:

	1. Insert A and B in a list (freeze_me_first_list)
	2. Have freezer scan freeze_me_first_list before the master
	   task-list, so that it:
		2a. "freezes A and waits for A to get frozen" first
		2b. "freezes B and waits for B to get frozen" next

then we would avoid the nastiness of "B getting frozen first and A doesnt
freeze because of that" with lesser code changes?

-- 
Regards,
vatsa
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