linux-kernel - Re: f_owner.lock and file->pos updates

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Date:	Wed, 7 Mar 2007 17:50:05 -0800
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: f_owner.lock and file->pos updates

On 3/7/07, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> The right way IMHO would be to do the work that was done for pread/pwrite
> and implement preadv/pwritev. The moment you want to do atomic things
> with the file->f_pos instead of doing it with a local passed pos value it
> gets ugly.. why do you need to do it with f_pos ?

I didn't see any clean way to intersperse overwrites and appends to a
record-structured file without using vfs_llseek, which steps on f_pos.
 Actually, we may already have a problem with append races in
sys_write/sys_writev.  If it's possible for two threads to write() to
the same file in different threads (both intending to append), they
may wind up passing the same "pos" value into vfs_write().  Or does
fget_light/fput_light do some sort of locking that I'm not seeing?

Cheers,
- Michael
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