Re: [PATCH] Describe race of direct read and fork for unaligned buffers

From: Nick Piggin
Date: Wed May 09 2012 - 03:01:38 EST

On 9 May 2012 15:35, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> wrote:
> On Wed, May 9, 2012 at 11:10 AM, Nick Piggin <npiggin@xxxxxxxxx> wrote:
>> On 6 May 2012 01:29, KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> 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.
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