linux-kernel - Re: [PATCH 12/25] sysfs: Introduce sysfs_rename

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Date:	Wed, 08 Aug 2007 17:19:32 +0900
From:	Tejun Heo <htejun@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Greg KH <greg@...ah.com>, linux-kernel@...r.kernel.org,
	satyam@...radead.org, cornelia.huck@...ibm.com,
	stern@...land.harvard.edu,
	Linux Containers <containers@...ts.osdl.org>, gregkh@...e.de
Subject: Re: [PATCH 12/25] sysfs: Introduce sysfs_rename_mutex

Hello, Eric.

Eric W. Biederman wrote:
> Looking carefully at the rename code we have a subtle dependency
> that the structure of sysfs not change while we are performing
> a rename.  If the parent directory of the object we are renaming
> changes while the rename is being performed nasty things could
> happen when we go to release our locks.
> 
> So introduce a sysfs_rename_mutex to prevent this highly
> unlikely theoretical issue.

Yeah, it's a theoretical issue.  Rename/move implementation has always
depended on the parent structure not changing beneath it, but it's nice
to tighten up loose ends.

> +DEFINE_MUTEX(sysfs_rename_mutex);

Probably doesn't really matter but wouldn't a rwsem fit better?

> @@ -774,7 +775,7 @@ static struct dentry *__sysfs_get_dentry(struct super_block *sb, struct sysfs_di
>   *	down from there looking up dentry for each step.
>   *
>   *	LOCKING:
> - *	Kernel thread context (may sleep)
> + *	mutex_lock(sysfs_rename_mutex)

LOCKING describes what locks should be held when entering the function,
so proper description would be something like...

	Kernel thread context, grabs sysfs_rename_mutex

Thanks.

-- 
tejun
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