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

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Message-Id: <20061130031933.5d30ec09.akpm@osdl.org>
Date:	Thu, 30 Nov 2006 03:19:33 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Ingo Molnar <mingo@...e.hu>
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

On Thu, 30 Nov 2006 12:03:15 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Gautham R Shenoy <ego@...ibm.com> wrote:
> 
> > a) cpufreq maintain's it's own cpumask in the variable 
> > policy->affected_cpus and says : If a frequency change is issued to 
> > any one of the cpu's in the affected_cpus mask, you change frequency 
> > on all cpus in the mask. So this needs to be consistent with 
> > cpu_online map and hence cpu hotplug aware. Furthermore, we don't want 
> > cpus in this mask to go down when we are trying to change frequencies 
> > on them. The function which drives the frequency change in 
> > cpufreq-core is cpufreq_driver_target and it needs cpu-hotplug 
> > protection.
> 
> couldnt this complexity be radically simplified by having new kernel 
> infrastructure that does something like:
> 
>   " 'gather' all CPUs mentioned in <mask> via scheduling a separate 
>     helper-kthread on every CPU that <mask> specifies, disable all
>    interrupts, and execute function <fn> once all CPUs have been 
>    'gathered' - and release all CPUs once <fn> has executed on each of
>    them."
> 
> ?

How does this differ from stop_machine_run(), which hot-unplug presently uses?

> 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.
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