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

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Message-ID: <20120502092029.GD16976@quack.suse.cz>
Date:	Wed, 2 May 2012 11:20:29 +0200
From:	Jan Kara <jack@...e.cz>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Nick Piggin <npiggin@...il.com>, mtk.manpages@...il.com,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	linux-man@...r.kernel.org, linux-mm@...ck.org, mgorman@...e.de,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [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@...il.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@...e.cz>
SUSE Labs, CR
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