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    SubjectRe: [RFC][PATCH] irq_work
    From: Andi Kleen <>
    Date: Thu, Jun 24, 2010 at 10:01:43AM -0400

    > > Please, as Peter and Boris asked you already, quote a concrete, specific
    > > example:
    > It was already in my answer to Peter.
    > >
    > > 'Specific event X occurs, kernel wants/needs to do Y. This cannot be done
    > > via the suggested method due to Z.'
    > >
    > > Your generic arguments look wrong (to the extent they are specified) and it
    > > makes it much easier and faster to address your points if you dont blur them
    > > by vagaries.
    > It's one of the fundamental properties of recoverable errors.
    > Error happens.
    > Machine check or NMI or other exception happens.
    > That exception runs on the exception stack
    > The error is not fatal, but recoverable.
    > For example you want to kill a process or call hwpoison or do some other
    > recovery action. These generally have to sleep to do anything
    > interesting.
    > You cannot do the sleeping on the exception stack, so you push it to
    > another context.
    > Now just because an error is recoverable doesn't mean it's not critical
    > (I think that was the mistake Boris made).

    It wasn't a mistake - I was simply trying to lure you into giving a more
    concrete example so that we all land on the same page and we know what
    the heck you/we/all are talking about.

    > If you don't do something
    > (like killing or recovery) you could end up in a loop or consume
    > corrupted data or something else bad.
    > So the error has to have a fail safe path from detection to handling.

    So we are talking about a more involved and "could-sleep" error

    > That's quite different from logging or performance counting etc.
    > where dropping events on overload is normal and expected.

    So I went back and reread the whole thread, and correct me if I'm
    wrong but the whole run softirq after NMI has one use case for now -
    "could-sleep" error handling for MCEs _only_ on x86. So you're changing
    a bunch of generic and x86 kernel code just for error handling. Hmm,
    that's a kinda big hammer in my book.

    A slimmer solution is a much better way to go, IMHO. I think Peter said
    something about irq_exit(), which should be just fine.

    But AFAICT an arch-specific solution would be even better, e.g.
    if you call into your deferred work helper from paranoid_exit in
    <arch/x86/kernel/entry_64.S>. I.e, something like

    #ifdef CONFIG_X86_MCE
    testl $_TIF_NEED_POST_NMI,%ebx
    jnz do_post_nmi_work

    Or even slimmer, rewrite the paranoidzeroentry to a MCE-specific variant
    which does the added functionality. But that wouldn't be extensible if
    other entities want post-NMI work later.


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