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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    On Wed, May 9, 2012 at 11:10 AM, Nick Piggin <> wrote:
    > On 6 May 2012 01:29, KOSAKI Motohiro <> wrote:
    >>> So, am I correct to assume that right text to add to the page is as below?
    >>> Nick, can you clarify what you mean by "quiesced"?
    >> finished?
    > Yes exactly. That might be a simpler word. Thanks!


    But see below. I realize the text is still ambiguous.

    >>> [[
    >>> O_DIRECT IOs should never be run concurrently with fork(2) system call,
    >>> when the memory buffer is anonymous memory, or comes from mmap(2)
    >>> with MAP_PRIVATE.
    >>> Any such IOs, whether submitted with asynchronous IO interface or from
    >>> another thread in the process, should be quiesced before fork(2) is called.
    >>> Failure to do so can result in data corruption and undefined behavior in
    >>> parent and child processes.
    >>> This restriction does not apply when the memory buffer for the O_DIRECT
    >>> IOs comes from mmap(2) with MAP_SHARED or from shmat(2).
    >>> Nor does this restriction apply when the memory buffer has been advised
    >>> as MADV_DONTFORK with madvise(2), ensuring that it will not be available
    >>> to the child after fork(2).
    >>> ]]

    In the above, the status of a MAP_SHARED MAP_ANONYMOUS buffer is
    unclear. The first paragraph implies that such a buffer is unsafe,
    while the third paragraph implies that it *is* safe, thus
    contradicting the first paragraph. Which is correct?



    Michael Kerrisk
    Linux man-pages maintainer;
    Author of "The Linux Programming Interface";

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