linux-kernel - Re: [RESEND] [PATCH] VFS: make file->f

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Message-ID: <20081008025209.GO25780@parisc-linux.org>
Date:	Tue, 7 Oct 2008 20:52:09 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	torvalds@...ux-foundation.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

On Wed, Oct 08, 2008 at 01:35:44PM +1100, Nick Piggin wrote:
> Right you are. That's the fundamental question. The actual details of
> the fix and how likely the race is don't really matter until we
> answer the first question (except to say that the "fix" is never going
> to be free).
> 
> We've lasted this long with the current semantics. So the natural
> reaction to anything that strengthens the semantics now is "why?". If
> we do that then we can basically never return to the weaker semantics.
> So there had better be a really good reason.

And it's worth saying that letter-of-the-standard arguments aren't
necessarily enough.  Linux does not honour the POSIX guarantee that
writes are atomic (if they cross page boundaries, it's not certain).
This seems like even more of a corner case to me.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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