Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

Hisashi Hifumi
2008-10-08T04:51:20Z

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.