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    SubjectRe: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic
    On Mon, 15 Jun 2009, Ingo Molnar wrote:
    >
    > Note that Oprofile uses the same method of __copy_user_inatomic() in
    > arch/x86/oprofile/backtrace.c, but i believe that code is broken - i
    > doubt the call-chain support for user-space stacks ever worked in
    > oprofile - with perfcounters i can make this method crash under
    > load. (we re-enter the NMI which due to IST executes over the exact
    > same, still pending NMI frame. Kaboom.)
    >
    > I saw you being involved with the Oprofile code 3 years ago:
    >
    > | commit c34d1b4d165c67b966bca4aba026443d7ff161eb
    > | Author: Hugh Dickins <hugh@veritas.com>
    > | Date: Sat Oct 29 18:16:32 2005 -0700
    > |
    > | [PATCH] mm: kill check_user_page_readable
    >
    > That method of __copy_user_inatomic(), while elegant, is subtly
    > wrong in an NMI context. We really must avoid taking faults there.

    Yes, I'm afraid that subtlety escaped me - thanks for explaining.

    Hugh


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