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

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Message-Id: <6.0.0.20.2.20081008132532.056cc400@172.19.0.2>
Date:	Wed, 08 Oct 2008 13:48:10 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	torvalds@...ux-foundation.org
Cc:	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


At 11:35 08/10/08, Nick Piggin wrote:
>On Wednesday 08 October 2008 05:59, Peter Zijlstra wrote:
>>
>> The whole point is that such usage is outside the specification and thus
>> we don't strictly need to fix this.
>>
>> So the question Nick is asking is, do we want to slow down the kernel
>> for a few broken user-space applications. Esp. since the race doesn't
>> affect anybody else except the broken users of the file descriptor.
>
>Right you are. That's the fundamental question. The actual details of
>the fix and how likely the race is don't really matter until we
>answer the first question (except to say that the "fix" is never going
>to be free).

Simultaneous access by two or more writer can corrupt file content,
so this case needs some locks(flock or fcntl) to preserve synchronization
of file content. This is responsibility of user-space application.
But file->f_pos race issue can occur even if multiple threads just read 
simultaneously. I think this is not responsibility of user-space application. 
To avoid this currently, an application needs some locks to protect file offset
even if it just read a file. So I think f_pos race should be fixed.

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