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

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Message-ID: <20061130143522.GA28507@elte.hu>
Date:	Thu, 30 Nov 2006 15:35:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, 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


* Gautham R Shenoy <ego@...ibm.com> wrote:

> 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?

that would be attached to a very cache-inefficient thing: migrating a 
task from one CPU to another. This is not the kind of ping-pong we are 
normally worried about. (nor does it happen all that often)

	Ingo
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