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

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Message-Id: <1223457442.1378.42.camel@lappy.programming.kicks-ass.net>
Date:	Wed, 08 Oct 2008 11:17:22 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	torvalds@...ux-foundation.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>, 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, 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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