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SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
On Tue 01-05-12 20:04:15, Hugh Dickins wrote:
> On Wed, 2 May 2012, Nick Piggin wrote:
> > On 2 May 2012 03:56, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
> > >
> > > In the light of all of the comments, can someone revise the man-pages
> > > patch that Jan sent?
> >
> > This does not quite describe the entire situation, but something understandable
> > to developers:
> >
> > 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).
Yes, I think with this addition the text is fine.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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