linux-kernel - Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

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Date:	Thu, 30 Nov 2006 18:14:21 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...l.org>, Gautham R Shenoy <ego@...ibm.com>,
	linux-kernel@...r.kernel.org, torvalds@...l.org, davej@...hat.com,
	dipankar@...ibm.com, vatsa@...ibm.com
Subject: Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

On Thu, Nov 30, 2006 at 12:46:17PM +0100, Ingo Molnar wrote:
> 
> 	struct task_struct {
> 		...
> 		int hotplug_depth;
> 		struct mutex *hotplug_lock;
> 	}
> 	...
> 
> 	DEFINE_PER_CPU(struct mutex, hotplug_lock);
> 
> 	void cpu_hotplug_lock(void)
> 	{
> 		int cpu = raw_smp_processor_id();
> 		/*
> 		 * Interrupts/softirqs are hotplug-safe:
> 		 */
> 		if (in_interrupt())
> 			return;
> 		if (current->hotplug_depth++)
> 			return;
> 		current->hotplug_lock = &per_cpu(hotplug_lock, cpu);
> 		mutex_lock(current->hotplug_lock);
> 	}
> 
> 	void cpu_hotplug_unlock(void)
> 	{
> 		int cpu;
> 
> 		if (in_interrupt())
> 			return;
> 		if (DEBUG_LOCKS_WARN_ON(!current->hotplug_depth))
> 			return;
> 		if (--current->hotplug_depth)
> 			return;
> 
> 		mutex_unlock(current->hotplug_lock);
> 		current->hotplug_lock = NULL;
> 	}
> 

In process context preemptible code, 
Lets say we are currently running on processor i.

cpu_hotplug_lock() ; /* does mutex_lock(&percpu(hotplug_lock, i)) */

/* do some operation, which might sleep */
/* migrates to cpu j */

cpu_hotplug_unlock(); /* does mutex_unlock(&percpu(hotplug_lock, i)
			 while running on cpu j */

This would cause cacheline ping pong, no?

> 
> 	Ingo

regards 
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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