Subject: Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit
	arch
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Eric Dumazet <dada1@cosmosbay.com>
Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>,
       Nick Piggin <nickpiggin@yahoo.com.au>, 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>
In-Reply-To: <48EC7003.4040108@cosmosbay.com>
References: <6.0.0.20.2.20081007140438.0580f110@172.19.0.2>
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	 <1223405963.26330.83.camel@lappy.programming.kicks-ass.net>
	 <200810081335.44576.nickpiggin@yahoo.com.au>
	 <6.0.0.20.2.20081008132532.056cc400@172.19.0.2>
	 <1223448711.1378.16.camel@lappy.programming.kicks-ass.net>
	 <48EC7003.4040108@cosmosbay.com>
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Date: Wed, 08 Oct 2008 11:17:22 +0200
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On Wed, 2008-10-08 at 10:32 +0200, Eric Dumazet wrote:

> About dup() syscall, it wont help, since old and new descriptor points to
> the same "struct file", definitly sharing file position, since first Unixes.
> 
> To quote the fine manual :
> 
>        After successful return of dup or dup2, the old and new descriptors may
>        be used interchangeably. They share locks, file position  pointers  and
>        flags;  for example, if the file position is modified by using lseek on
>        one of the descriptors, the position is also changed for the other.

Ah, ok. I'll try to remember for next time I write a multi-threaded user
app (which given the size of my kernel todo list won't be any time soon
I guess ;-)

> pread()/pwrite() are used my multi-threaded applications that want to share
> a single "struct file".

Yeah, pread/pwrite is good.

>  Or they must use some form of synchronization around
> regular read()/write()/lseek() calls.
> 
> There is no generic f_pos race, only buggy applications.

Agreed.


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