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

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Date:	Wed, 08 Aug 2007 17:23:11 +0900
From:	Tejun Heo <teheo@...e.de>
To:	Tejun Heo <htejun@...il.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	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

Tejun Heo wrote:
> 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

Oops, forget about the above.  Thought the comment was added to
sysfs_rename_dir().

-- 
tejun
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