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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    On 9 May 2012 15:35, Michael Kerrisk (man-pages) <> wrote:
    > 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!
    > 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?

    Yes I see. It's because MAP_SHARED | MAP_ANONYMOUS isn't *really*
    anonymous from the virtual memory subsystem's point of view. But that
    just serves to confuse userspace I guess.

    Anything with MAP_SHARED, shmat, or MADV_DONTFORK is OK.

    Anything else (MAP_PRIVATE, brk, without MADV_DONTFORK) is
    dangerous. These type are used by standard heap allocators malloc,
    new, etc.

     \ /
      Last update: 2012-05-09 09:41    [W:0.027 / U:88.404 seconds]
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