linux-kernel - Re: [RFD] Freezing of kernel threads

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Date:	Mon, 14 May 2007 12:07:21 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	ego@...ibm.com
Cc:	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>, Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Paul E McKenney <paulmck@...ibm.com>
Subject: Re: [RFD] Freezing of kernel threads

On Monday, 14 May 2007 09:26, Gautham R Shenoy wrote:
> On Mon, May 14, 2007 at 11:48:46AM +0530, Srivatsa Vaddagiri wrote:
> > 
> > The other complication get/put_hotcpu() had was dealing with
> > write-followed-by-read lock attempt by the *same* thread (whilst doing
> > cpu_down/up).  IIRC this was triggered by some callback processing in CPU_DEAD 
> > or CPU_DOWN_PREPARE.
> > 
> > 
> > cpu_down()
> >  |- take write lock 
> >  |- CPU_DOWN_PREPARE
> >  |        |- foo() wants a read_lock 
> > 
> > Stupid as it sounds, it was really found to be happening!  Gautham, do you 
> > recall who that foo() was? Somebody in cpufreq I guess ..
> 
> IIRC, it was a problem with ondemand. while handling CPU_DEAD, ondemand code
> would call destroy_workqueue, which tried flushing the workqueue, which
> once upon a time did lock_cpu_hotplug, before Oleg and Andrew cleaned 
> that up. 
> 
> Ofcourse, cpufreq works fine now after Venki's patches which
> just nullifies the reference to the policy structure of the cpu to be
> removed during the CPU_DOWN_PREPARE by calling __cpufreq_remove_dev
> instead of handling it in CPU_DEAD.
> 
> However, as we have discovered, without freezing all the threads, it
> is inadvisable to call flush_workqueue from a cpu-hotplug callback 
> path. 

Please see my recent patch at http://lkml.org/lkml/2007/5/14/7 .
It's not exactly the same thing, but I think the trick in there might be
useful.

Greetings,
Rafael
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