linux-kernel - Re: PCI PM: Restore standard config registers of all devices early

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Date:	Tue, 3 Feb 2009 06:06:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	Andreas Schwab <schwab@...e.de>, Len Brown <lenb@...nel.org>
Subject: Re: PCI PM: Restore standard config registers of all devices early


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, 3 Feb 2009, Benjamin Herrenschmidt wrote:
> > 
> > IE, you should have something to ensure, before you turn interrupts off,
> > that nobody else is inside the AML interpreter. You already know there
> > are no other CPUs, so it's just a matter of making sure no other process
> > has scheduled while holding that mutex.
> > 
> > The easy way to do that is to do something like taking the mutex
> > yourself and then setting a flag so that the intepreter stops trying to
> > take it or release it itself, maybe just using the global system state.
> > 
> > Then release the mutex on resume.
> 
> Why do you think this improves on anything?
> 
> Basically, it turns the mutex into a non-entity - but if your whole 
> argument is that it might as well be a non-entity because nobody else can 
> take it anyway, then why not just leave it around?
> 
> IOW, if your argument boils down to "there can be no contention", then you 
> might as well say "just use the mutex, it will never block".
> 
> So the only thing you really need is to just disable the _debugging_ code 
> that mutexes have (if they get built with debugging in the first place).
> 
> I can't find the bothersome code anyway: I do find
> 
> 	DEBUG_LOCKS_WARN_ON(in_interrupt());
> 
> but that's just saying that you shouldn't be using mutexes from 
> interrupts, not from irq-off segments. There's probably something I'm 
> missing, like the preempt_check_resched() causing a schedule event with 
> irq's disabled, and the "might_sleep()" thing. But the latter should 
> already be disabled by the "system_state != SYSTEM_RUNNING" thing.

Mutexes should work just fine in irqs-off sections - they'll safely 
save/restore interrupts, even the debug variants.

We used to have code in the mutex code that unconditionally enabled 
interrupts (a spin_unlock_irq() iirc) - but we fixed that pretty
early on because it surprised some early boot code. Maybe this is
the case you remember?

	Ingo
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