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    SubjectRe: [PATCH v3 5/5] irqchip: gic: Add support for IPI FIQ
    On 09/09/14 09:24, Daniel Thompson wrote:
    > On 08/09/14 17:23, Russell King - ARM Linux wrote:
    >> On Mon, Sep 08, 2014 at 04:28:35PM +0100, Daniel Thompson wrote:
    >>> @@ -604,8 +731,19 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
    >>> {
    >>> int cpu;
    >>> unsigned long flags, map = 0;
    >>> + unsigned long softint;
    >>>
    >>> - raw_spin_lock_irqsave(&irq_controller_lock, flags);
    >>> + /*
    >>> + * The locking in this function ensures we don't use stale cpu mappings
    >>> + * and thus we never route an IPI to the wrong physical core during a
    >>> + * big.LITTLE switch. The switch code takes both of these locks meaning
    >>> + * we can choose whichever lock is safe to use from our current calling
    >>> + * context.
    >>> + */
    >>> + if (in_nmi())
    >>> + raw_spin_lock(&fiq_safe_migration_lock);
    >>> + else
    >>> + raw_spin_lock_irqsave(&irq_controller_lock, flags);
    >>
    >> Firstly, why would gic_raise_softirq() be called in FIQ context?
    >
    > Oops.
    >
    > This code should have been removed. It *is* required for kgdb (which
    > needs to send FIQ to other processors via IPI and may itself be running
    > from FIQ) but it not needed for the currently targeted use case.

    I'm afraid I was wrong about this. gic_raise_softitq() is called during
    console unlocking inside wake_up_klogd(). This means it is required even
    to support arch_trigger_all_cpu_backtrace.

    I'm trying to get a (tested) refresh of the FIQ + trigger_backtrace out
    today. Thus for now I plan to reinstate the code above (which I believe
    to be safe because FIQ is disabled throughout a b.L switch).

    Nevertheless I won't ignore this comment! I think a using a r/w lock
    here can be made FIQ-safe without having to rely on in_nmi() based
    conditional branches.


    Daniel.


    >> Secondly,
    >> this doesn't save you. If you were in the middle of gic_migrate_target()
    >> when the FIQ happened that (for some reason prompted you to call this),
    >> you would immediately deadlock trying to that this IRQ.
    >
    > This cannot happen because gic_migrate_target() runs with FIQ disabled.
    >
    >
    >> I suggest not even trying to solve this "race" which I don't think is
    >> one which needs to even be considered (due to the first point.)
    >
    > As mentioned above I believe it eventually needs to be addressed by some
    > means but it certainly doesn't belong in the current patchset.
    >
    > I will remove it.
    >



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