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    SubjectRe: irq lock inversion

    * Tejun Heo <> wrote:

    > Ingo Molnar wrote:
    > >>> This warning is bogus -- sched_init() is being called very early with IRQs
    > >>> disabled, and the irqsave/restore code paths in pcpu_alloc() are only for early
    > >>> init. The path can never be called from irq context once the early init
    > >>> finishes. Rationale for this is explained in changelog of the commit mentioned
    > >>> above.
    > >>>
    > >>> This problem can be encountered generally in any other early code running
    > >>> with IRQs off and using irqsave/irqrestore.
    > >>>
    > >>> Reported-by: Yinghai Lu <>
    > >>> Signed-off-by: Jiri Kosina <>
    > >> Looks good to me. Ingo, what do you think?
    > >
    > > Ugh, this explanation is _BOGUS_. As i said, taking a lock with irqs
    > > disabled does _NOT_ mark a lock as 'irq safe' - if it did, we'd have
    > > false positives left and right.
    > >
    > > Read the lockdep message please, consider all the backtraces it prints,
    > > it says something different.
    > Ah... okay, the pcpu_free() path is correctly marking the lock
    > irqsafe. I assumed this was caused by recent pcpu_alloc() change.
    > Sorry about that. The lock inversion problem has always been there,
    > it just never showed up because none has use allocation map that large
    > I suppose.
    > So, the correct fix would be either 1. push down irqsafeness down to
    > vmalloc locks or 2. the rather ugly unlock-lock dancing in
    > pcpu_extend_area_map() I posted earlier. For 2.6.32, I guess we'll
    > have to go with #2. For longer term, we'll probably have to do #1 as
    > it's required to implement atomic percpu allocations too.
    > I'll try to reproduce the problem here and verify the previous locking
    > dance patch.

    I havent looked deeply but at first sight i'm not 100% sure that even
    the lock dance hack is safe - doesnt vfree() do TLB flushes, which must
    be done with irqs enabled in general? If yes, then the whole notion of
    using the allocator from irqs-off sections is wrong and the flags
    save/restore is misguided (or at least incomplete).

    So the real problem right now i think is the use of the pcpu allocator
    from within a BH section (and from irqs-off sections) - that usage
    should be eliminated from .32, or the allocator should be fixed. (which
    looks non-trivial vmalloc/vfree was never really intended to be used in
    irq-atomic contexts)


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