linux-kernel - Re: workqueue deadlock

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Date:	Sun, 10 Dec 2006 13:19:15 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...l.org>
Cc:	vatsa@...ibm.com, 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


* Andrew Morton <akpm@...l.org> wrote:

> This is actually not cpu-hotplug safe ;)  
> 
> > > > 	{
> > > > 		int cpu = raw_smp_processor_id();
> > > > 		/*
> > > > 		 * Interrupts/softirqs are hotplug-safe:
> > > > 		 */
> > > > 		if (in_interrupt())
> > > > 			return;
> > > > 		if (current->hotplug_depth++)
> > > > 			return;
> 
> <preempt, cpu hot-unplug, resume on different CPU>
> 
> > > > 		current->hotplug_lock = &per_cpu(hotplug_lock, cpu);
> 
> <use-after-free>
> 
> > > > 		mutex_lock(current->hotplug_lock);
> 
> And it sleeps, so we can't use preempt_disable().

i explained it in the other mail - this is the 'read' side. The 'write' 
side (code actually wanting to /do/ a CPU hotplug state transition) has 
to take /all/ these locks before it can take a CPU down.

so this is still a global CPU hotplug lock, but made scalable.

	Ingo
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