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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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