linux-kernel - Re: [PATCH] sysfs: only allow one scheduled removal callback per kobj

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Date:	Mon, 16 Mar 2009 11:15:42 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Alex Chiang <achiang@...com>
Cc:	Greg KH <gregkh@...e.de>, Tejun Heo <tj@...nel.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>, jbarnes@...tuousgeek.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysfs: only allow one scheduled removal callback per
 kobj

On Fri, 13 Mar 2009 12:07:36 -0600,
Alex Chiang <achiang@...com> wrote:

> The only way for a sysfs attribute to remove itself (without
> deadlock) is to use the sysfs_schedule_callback() interface.
> 
> Vegard Nossum discovered that a poorly written sysfs ->store
> callback can repeatedly schedule remove callbacks on the same
> device over and over, e.g.
> 
> 	$ while true ; do echo 1 > /sys/devices/.../remove ; done
> 
> If the 'remove' attribute uses the sysfs_schedule_callback API
> and also does not protect itself from concurrent accesses, its
> callback handler will be called multiple times, and will
> eventually attempt to perform operations on a freed kobject,
> leading to many problems.
> 
> Instead of requiring all callers of sysfs_schedule_callback to
> implement their own synchronization, provide the protection in
> the infrastructure.
> 
> Now, sysfs_schedule_callback will only allow one scheduled
> callback per kobject. On subsequent calls with the same kobject,
> return -EAGAIN.
> 
> This is a short term fix. The long term fix is to allow sysfs
> attributes to remove themselves directly, without any of this
> callback hokey pokey.
> 
> [cornelia.huck@...ibm.com: s390 ccwgroup bits]
> Cc: cornelia.huck@...ibm.com
> Reported-by: vegard.nossum@...il.com
> Signed-off-by: Alex Chiang <achiang@...com>
> ---
>  drivers/s390/cio/ccwgroup.c |    5 +++--
>  fs/sysfs/file.c             |   26 +++++++++++++++++++++++---
>  2 files changed, 26 insertions(+), 5 deletions(-)

Acked-by: Cornelia Huck <cornelia.huck@...ibm.com>
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