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Date: Sun, 10 Dec 2006 13:19:15 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@osdl.org>
Cc: vatsa@in.ibm.com, Bjorn Helgaas <bjorn.helgaas@hp.com>,
       linux-kernel@vger.kernel.org, Myron Stowe <myron.stowe@hp.com>,
       Jens Axboe <axboe@kernel.dk>, Dipankar <dipankar@in.ibm.com>,
       Gautham shenoy <ego@in.ibm.com>
Subject: Re: workqueue deadlock
Message-ID: <20061210121914.GA20466@elte.hu>
References: <20061207105148.20410b83.akpm@osdl.org> <20061207113700.dc925068.akpm@osdl.org> <20061208025301.GA11663@in.ibm.com> <20061207205407.b4e356aa.akpm@osdl.org> <20061209102652.GA16607@elte.hu> <20061209114723.138b6e89.akpm@osdl.org> <20061210082616.GB14057@elte.hu> <20061210004318.8e1ef324.akpm@osdl.org> <20061210114943.GA14749@elte.hu> <20061210041600.56306676.akpm@osdl.org>
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* Andrew Morton <akpm@osdl.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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