Date: Thu, 30 Nov 2006 17:49:01 +0530
From: Gautham R Shenoy <ego@in.ibm.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Gautham R Shenoy <ego@in.ibm.com>, akpm@osdl.org,
       linux-kernel@vger.kernel.org, torvalds@osdl.org, davej@redhat.com,
       dipankar@in.ibm.com, vatsa@in.ibm.com, paulmck@us.ibm.com
Subject: Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency
Message-ID: <20061130121901.GA25439@in.ibm.com>
Reply-To: ego@in.ibm.com
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On Thu, Nov 30, 2006 at 12:53:27PM +0100, Ingo Molnar wrote:
> 
> * Gautham R Shenoy <ego@in.ibm.com> wrote:
> 
> > This is what is currently being done by cpufreq:
> 
> ok!
> 
> > a) get_some_cpu_hotplug_protection() [use either some global mechanism
> > 					or a persubsystem mutex]
> 
> this bit is wrong i think. Any reason why it's not a per-CPU (but
> otherwise global) array of mutexes that controls CPU hotplug - as per my
> previous mail?
> 
> that would flatten the whole locking. Only one kind of lock taken,
> recursive and scalable.

I had posted one such recursive scalable version which can be found here
http://lkml.org/lkml/2006/10/26/73

I remember cc'ing you.

Yeah, it looks complicated and big, but then I did not want to add
another field to the task struct as one such attempt had already been
frowned upon ( I think long back Ashok posted it)

So I ended up writing the whole read/write lock/unlock code myself.

It's a RCU based lock, extremely light on the read side, but costly for the
writers since it does a synchronize_sched.

And yeah, it's partial towards the readers but with an additional field
in the task struct we can have a fair implementation.

Besides, an unfair cpu_hotplug_lock won't work since a process doing a
sched_getaffinity in a forever_while loop can prevent any hotplug from
happening.

> 
> Then the mechanism that changes CPU frequency should take all these
> hotplug locks on all (online) CPUs, and then first stop all processing
> on all CPUs, and then do the frequency change, atomically. This is with
> interrupts disabled everywhere /first/, and /without any additional
> locking/. That would prevent any sort of interaction from other CPUs -
> they'd all be sitting still with interrupts disabled.
> 

Yup.

> 	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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