Date: Thu, 30 Nov 2006 15:35:22 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Gautham R Shenoy <ego@in.ibm.com>
Cc: Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org,
       torvalds@osdl.org, davej@redhat.com, dipankar@in.ibm.com,
       vatsa@in.ibm.com
Subject: Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency
Message-ID: <20061130143522.GA28507@elte.hu>
References: <20061129152404.GA7082@in.ibm.com> <20061130083144.GC29609@elte.hu> <20061130102410.GB23354@in.ibm.com> <20061130110315.GA30460@elte.hu> <20061130031933.5d30ec09.akpm@osdl.org> <20061130114617.GA2324@elte.hu> <20061130124421.GB25439@in.ibm.com>
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* Gautham R Shenoy <ego@in.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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