linux-kernel - Re: sys_write() racy for multi-threaded append?

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Date:	Fri, 09 Mar 2007 01:15:24 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"Michael K. Edwards" <medwards.linux@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: sys_write() racy for multi-threaded append?

Michael K. Edwards a écrit :
> On 3/8/07, Eric Dumazet <dada1@...mosbay.com> wrote:
>> Nothing in the manuals says that write() on same fd should be non racy 
>> : In
>> particular file pos might be undefined. There is a reason pwrite() 
>> exists.
>>
>> Kernel doesnt have to enforce thread safety as standard is quite clear.
> 
> I know the standard _allows_ us to crash and burn (well, corrupt
> f_pos) when two threads race on an fd, but why would we want to?
> Wouldn't it be better to do something at least slightly sane, like add
> atomically to f_pos the expected number of number of bytes written,
> then do the write, then fix it up (again atomically) if vfs_write
> returns an unexpected pos?

Absolutely not. We dont want to slow down kernel 'just in case a fool might 
want to do crazy things'


> 
>> Only O_APPEND case is specially handled (and NFS might fail to handle 
>> this
>> case correctly)
> 
> Is it?  How?
mm/filemap.c

generic_write_checks()

if (file->f_flags & O_APPEND)
	*pos = i_size_read(inode);

done while inode is locked.

O_APPEND basically says : Just ignore fpos and always use the 'end of file'

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