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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: dcg <diegocalleja@gmail.com>
Subject: Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch
Date: Fri, 10 Oct 2008 13:25:15 +1100
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Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>,
       torvalds@linux-foundation.org,
       Andrew Morton <akpm@linux-foundation.org>,
       Andi Kleen <andi@firstfloor.org>, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org,
       "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
       "Theodore Ts'o" <tytso@mit.edu>
References: <6.0.0.20.2.20081007140438.0580f110@172.19.0.2> <1223448711.1378.16.camel@lappy.programming.kicks-ass.net> <20081009235158.7d328aa0@diego-desktop>
In-Reply-To: <20081009235158.7d328aa0@diego-desktop>
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On Friday 10 October 2008 08:51, dcg wrote:
> El Wed, 08 Oct 2008 08:51:51 +0200, Peter Zijlstra <a.p.zijlstra@chello.nl> 
escribió:
> > either dup() the fd or open() the file twice. There is absolutely no
> > valid reason to have two threads read from the same fd without
> > synchronising their access to it - never.
>
> In case this is the final consensus, I think that a topic that is brought
> to the list every few months and even generates (aparently not neccesary)
> patches is a hint that there should be somewhere a commentary (*) like
> this:
>
> (*) I don't know if what I wrote is 100% correct.
>
>
> Signed-off-by: Diego Calleja García <diegocg@gmail.com>
>
> Index: 2.6/include/linux/fs.h
> ===================================================================
> --- 2.6.orig/include/linux/fs.h	2008-10-09 00:06:50.000000000 +0200
> +++ 2.6/include/linux/fs.h	2008-10-09 00:29:03.000000000 +0200
> @@ -821,6 +821,18 @@
>  	atomic_long_t		f_count;
>  	unsigned int 		f_flags;
>  	mode_t			f_mode;
> +	/*
> +	 * Linux does NOT guarantee atomic reading/writing to file->f_pos in
> +	 * multithread apps running in 32 bit machines. There're several
> +	 * reasons for this behaviour:

Note that I don't think we'd want to explicitly guarantee that it is atomic
on 64-bit machines either. It does happen to be, but I don't think we want
anybody to rely on that...
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