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

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Message-ID: <f2b55d220703081645vc78905cj56afbc58ad2113d8@mail.gmail.com>
Date:	Thu, 8 Mar 2007 16:45:41 -0800
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Eric Dumazet" <dada1@...mosbay.com>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: sys_write() racy for multi-threaded append?

On 3/8/07, Eric Dumazet <dada1@...mosbay.com> wrote:
> Absolutely not. We dont want to slow down kernel 'just in case a fool might
> want to do crazy things'

Actually, I think it would make the kernel (negligibly) faster to bump
f_pos before the vfs_write() call.  Unless fget_light sets fput_needed
or the write doesn't complete cleanly, you won't have to touch the
file table entry again after vfs_write() returns.  You can adjust
vfs_write to grab f_dentry out of the file before going into
do_sync_write.  do_sync_write is done with the struct file before it
goes into the aio_write() loop.  Result: you probably save at least an
L1 cache miss, unless the aio_write loop is so frugal with L1 cache
that it doesn't manage to evict the struct file.

Patch to follow.

Cheers,
- Michael
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