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

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Date:	Mon, 28 Jan 2008 14:50:47 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andi Kleen <ak@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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 15:10:34 +0100
Andi Kleen <ak@...e.de> wrote:

> On Monday 28 January 2008 14:38:57 Alan Cox wrote:
> > > Also worse really fixing it would be a major change to the VFS 
> > > because of the way ->read/write are defined :/
> > 
> > I don't see a problem there. ->read and ->write update the passed pointer
> > which is not the real f_pos anyway. Just the copies need fixing. 
> 
> They are effectually doing a decoupled read/modify/write cycle. e.g.:
> 
> A               B
> 
> read fpos       
> 
>                 read fpos
> 
> fpos += A       fpos += B
>                 write fpos
> 
> 
> write fpos
> 
> So you get overlapping reads. Probably not good.

No unix system I'm aware of cares about the read/write positioning during
parallel simultaneous reads or writes, with the exception of O_APPEND
which is strictly defined. The problem case is getting fpos != either
valid value.

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