Re: [BUG] bugs in jbd2_dev_to_name() (was Re: [PATCH 00/11] [GIT PULL] more updates for the tag format)

[Li Zefan <lizf@cn.fujitsu.com>]

Theodore Tso wrote:
> On Fri, Jun 19, 2009 at 04:14:23PM +0800, Li Zefan wrote:
>>> 	rcu_read_lock();
>>> 	if (devcache[i] && devcache[i]->device == device) {
>>> 		ret = devcache[i]->devname;
>>> 		rcu_read_unlock();
>>> 		return ret;
>> It doesn't seem safe to dereference @ret outside rcu read section.
> 
> Note the comments at the beginning of the function:
> 

Ah, I overlooked the comments.

But the patch that adds rcu locking around trace event prints
never gets merged:

http://lkml.org/lkml/2009/4/15/549

Steven?

>     The caller should use rcu_read_lock() in order to make sure the
>     device name stays valid until its done with it.  We use
>     rcu_read_lock() as well to make sure we're safe in case the caller
>     gets sloppy, and because rcu_read_lock() is cheap and can be safely
>     nested.
> 
> I suppose I should change the wording to indicate that it adds a bit
> more safety (as in, the crash won't happen inside this function, but
> as far as the caller is concerned, all bets are off!)
>