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

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Message-ID: <45F09F9C.4030801@cosmosbay.com>
Date:	Fri, 09 Mar 2007 00:43:24 +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 :
> from sys_write():
> 
>        file = fget_light(fd, &fput_needed);
>        if (file) {
>                loff_t pos = file_pos_read(file);
>                ret = vfs_write(file, buf, count, &pos);
>                file_pos_write(file, pos);
>                fput_light(file, fput_needed);
>        }
> 
> Surely that's racy when two threads write to the same fd and the first
> vfs_write call blocks or is preempted.  Or does fget_light take some
> per-file lock that I'm not seeing?

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.

Only O_APPEND case is specially handled (and NFS might fail to handle this 
case correctly)

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