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

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Date:	Thu, 30 Nov 2006 12:46:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...l.org>
Cc:	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


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

> > This would be done totally serialized and while holding the hotplug 
> > lock, so no CPU could go away or arrive while this operation is 
> > going on.
> 
> You said "the hotplug lock".  That is the problem.

maybe i'm too dense today but i still dont see the fundamental problem. 

Even with complex inter-subsystem interactions, hotplugging could be 
effectively and scalably controlled via a self-recursive per-CPU mutex, 
and a pointer to it embedded in task_struct:

	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;
	}

	...

	void do_exit(void)
	{
	...
		DEBUG_LOCKS_WARN_ON(current->hotplug_depth);
	...
	}
	...
	copy_process(void)
	{
	...
		p->hotplug_depth = 0;
		p->hotplug_lock = NULL;
	...
	}

50 lines of code at most. The only rule is to not use cpu_hotplug_lock() 
in process-context non-preemptible code [interrupt contexts are 
automatically ignored]. What am i missing?

	Ingo
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