Date: Thu, 16 Nov 2006 16:56:35 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: Thomas Gleixner <tglx@timesys.com>
cc: LKML <linux-kernel@vger.kernel.org>, Linus Torvalds <torvalds@osdl.org>,
       john stultz <johnstul@us.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       David Miller <davem@davemloft.net>,
       Arjan van de Ven <arjan@infradead.org>, Andrew Morton <akpm@osdl.org>,
       Andi Kleen <ak@suse.de>
Subject: Re: BUG: cpufreq notification broken
In-Reply-To: <1163712994.10333.49.camel@localhost.localdomain>
Message-ID: <Pine.LNX.4.44L0.0611161654270.2460-100000@iolanthe.rowland.org>
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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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