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

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Date:	Wed, 08 Aug 2007 02:28:52 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Tejun Heo <htejun@...il.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

Tejun Heo <htejun@...il.com> writes:

> 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?

Maybe.  I didn't feel any loss in when I was writing the code.
Very few code paths actually seem to care.

>> @@ -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)

Well this is weird in that it should be on sysfs_get_dentry
more then __sysfs_get_dentry but otherwise it's ok.

> 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

For rename_dir and move_dir yes.  I was updating the rules
for sysfs_get_dentry.  Which really wants it's parents to
hold that lock.

Eric
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