linux-kernel - Re: [PATCH] [8/18] BKL-removal: Remove BKL from remote

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Date:	Sun, 27 Jan 2008 21:13:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <ak@...e.de>
Cc:	Trond Myklebust <Trond.Myklebust@...app.com>,
	Steve French <smfrench@...il.com>, swhiteho@...hat.com,
	sfrench@...ba.org, vandrove@...cvut.cz,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] [8/18] BKL-removal: Remove BKL from remote_llseek

On Mon, 28 Jan 2008 05:38:25 +0100 Andi Kleen <ak@...e.de> wrote:

> On Monday 28 January 2008 05:13:09 Trond Myklebust wrote:
> > 
> > On Mon, 2008-01-28 at 03:58 +0100, Andi Kleen wrote:
> > > The problem is that it's not a race in who gets to do its thing first, but a 
> > > parallel reader can actually see a corrupted value from the two independent 
> > > words on 32bit (e.g. during a 4GB). And this could actually completely corrupt 
> > > f_pos when it happens with two racing relative seeks or read/write()s
> > > 
> > > I would consider that a bug.
> > 
> > I disagree. The corruption occurs because this isn't a situation that is
> > allowed by either POSIX or SUSv2/v3. Exactly what spec are you referring
> > to here?
> 
> No specific spec, just general quality of implementation.

I completely agree.  If one thread writes A and another writes B then the
kernel should record either A or B, not ((A & 0xffffffff00000000) | (B &
0xffffffff))
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