linux-kernel - Re: BUG: cpufreq notification broken

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Date:	Thu, 16 Nov 2006 16:56:35 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Thomas Gleixner <tglx@...esys.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	john stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>
Subject: Re: BUG: cpufreq notification broken

On Thu, 16 Nov 2006, Thomas Gleixner wrote:

> On Thu, 2006-11-16 at 16:26 -0500, Alan Stern wrote:
> > On Thu, 16 Nov 2006, Thomas Gleixner wrote:
> > 
> > > There is another issue with this SRCU change:
> > > 
> > > The notification comes actually after the real change, which is bad. We
> > > try to make the TSC usable by backing it with pm_timer accross such
> > > states, but this behaviour breaks the safety code.
> > 
> > I don't understand.  Sending notifications is completely separate from 
> > setting up the notifier chain's head.  The patch you mentioned didn't 
> > touch the code that sends the notifications.
> 
> Yeah, my bad. It just uses rcu based locking, but its still synchronous.
> 
> I have to dig deeper, why the change of the frequency happens _before_
> the notifier arrives.

There are supposed to be _two_ notifier calls: one before the frequency 
change and one after.  Check the callers of cpufreq_notify_transition().

Alan Stern

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