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    SubjectRe: [uml-devel] Re: [PATCH 07/10] uml: avoid fixing faults while atomic
    Blaisorblade <> wrote:
    > On Wednesday 21 September 2005 21:49, Andrew Morton wrote:
    > > "Paolo 'Blaisorblade' Giarrusso" <> wrote:
    > > > From: Paolo 'Blaisorblade' Giarrusso <>
    > > The in_atomic() test in x86's do_page_fault() is in fact a message passed
    > > into it from filemap.c's kmap_atomic().
    > Ok, this can be ok, but:
    > > It has accidental side-effects,
    > > such as making copy_to_user() fail if inside spinlocks when
    > Sorry, but should it ever succeed inside spinlocks? I mean, should it ever
    > call down() inside spinlocks? (We never do down_trylock, and ever if we did
    > the x86 trick, that wouldn't make the whole thing safe at all - they still
    > take the spinlock and potentially sleep. And it's legal only if no spinlock
    > is held).

    Not sure what you're asking here.

    copy_to/from_user() will fail inside spinlock if CONFIG_PREMPT=y and if the
    copy happens to cause a fault. Otherwise it will succeed inside spinlock,
    and it won't spew a sleeping-while-atomic warning, because that uses
    in_atomic() too. It might deadlock if we schedule away and try to retake
    the same lock.

    > Even if spinlocks don't always trigger in_atomic() - which means that we'd
    > need to have a better fix for this.

    The patch you have will correctly cause copy_*_user()->pagefault to fail
    the copy if the caller has run inc_preempt_count(). It will not cause
    copy_*_user()->pagefault to fail inside spinlocks unless UML does
    inc_preempt_count() in its spinlock implementation.

    > (Btw, I took the above reasoning from something said, as an aside, on
    > kernel page, about the FUTEX deadlock on mm->mmap_sem of ~ 2.6.8 - yes, it
    > wasn't the full truth, but not totally dumb).
    > > So I think this change is only needed if UML implements kmap_atomic, as in
    > > arch/i386/mm/highmem.c, which it surely does not do?
    > NACK, see above.

    Yup, the patch is needed for the futex code, and for general correct
    implementation of inc_preempt_count()'s intended effect.

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      Last update: 2005-09-21 22:51    [W:0.023 / U:130.520 seconds]
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