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

    * 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


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