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Date: Sat, 22 Jul 2006 17:10:36 -0700 (PDT)
From: Linus Torvalds <torvalds@osdl.org>
To: Dave Jones <davej@redhat.com>
cc: Andrew Morton <akpm@osdl.org>, Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: remove cpu hotplug bustification in cpufreq.
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On Sat, 22 Jul 2006, Dave Jones wrote:
>
> This stuff makes my head hurt. Someone who is motivated
> enough to fix up hotplug-cpu can fix this up later.
> In the meantime, this patch should cure the lockdep
> warnings that seem to trigger very easily.

It doesn't seem to fix all problems. On CPU unplug, I still get deadlocks 
through some workqueue:

 [<c03af64a>] __mutex_lock_slowpath+0x4d/0x7b
 [<c03af687>] .text.lock.mutex+0xf/0x14
 [<c0137591>] __lock_cpu_hotplug+0x36/0x56
 [<c01375ca>] lock_cpu_hotplug+0xa/0xc
 [<c012f7c2>] flush_workqueue+0x2d/0x61
 [<c012f83b>] flush_scheduled_work+0xd/0xf
 ...

and the nasty part is that this can apparently hit _any_ process that 
wants to flush workqueues (in one particular case, it was through 
tty_release() -> release_dev() in drivers/char/tty.c).

The whole CPU hotplug locking seems to be broken.

		Linus