linux-kernel - Re: workqueue deadlock

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Date:	Mon, 11 Dec 2006 10:28:31 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...l.org>,
	Bjorn Helgaas <bjorn.helgaas@...com>,
	linux-kernel@...r.kernel.org, Myron Stowe <myron.stowe@...com>,
	Jens Axboe <axboe@...nel.dk>, Dipankar <dipankar@...ibm.com>,
	Gautham shenoy <ego@...ibm.com>
Subject: Re: workqueue deadlock

On Sun, Dec 10, 2006 at 09:26:16AM +0100, Ingo Molnar wrote:
> something like the pseudocode further below - when applied to a data
> structure it has semantics and scalability close to that of
> preempt_disable(), but it is still preemptible and the lock is specific.

Ingo,
	The psuedo-code you have provided can still fail to avoid
the deadlock reported by Bjorn Helgaas earlier in this thread:

	http://lkml.org/lkml/2006/12/6/352

Thread1->flush_workqueue->mutex_lock(cpu4's hotplug_lock)

Thread2(keventd)->run_workqueue->som_work_fn-> ..
		flush_workqueue->mutex_lock(cpu4's hotplug_lock)

Both deadlock with each other.

All this mess could easily be avoided if we implement a reference-count
based cpu_hotplug_lock(), as suggested by Arjan and Linus before and
implemented by Gautham here:

	http://lkml.org/lkml/2006/10/26/65

-- 
Regards,
vatsa
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