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

Ingo Molnar
2009-02-03T05:07:39Z

* Linus Torvalds <torvalds@linux-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