linux-kernel - Re: Lockdep false positive in sysfs

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Message-ID: <20120426221753.GF27486@google.com>
Date:	Thu, 26 Apr 2012 15:17:53 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Lockdep false positive in sysfs

On Thu, Apr 26, 2012 at 02:14:30PM -0400, Alan Stern wrote:
> > > Hmmm.... This happens because, by default, sysfs_dirents for the same
> > > attr share the same lockdep key.  This happens from
> > > sysfs_dirent_init_lockdep().  Hmm.... we can,
> > >
> > > * Somehow assign different keys to sysfs_dirents for the specific
> > >   attr.  Use array of attrs indexed by bus depth?
> > 
> > Possible with sysfs_attr_init but pretty ugly.  Especially since it
> > sounds like this is a situation that does not presuppose a maximum
> > depth.  I do remember that the lockdep keys must be statically allocated
> > which makes this a challenge.

The depth is limited by USB spec.

> I agree; this doesn't seem like a good approach.

It sure isn't pretty but probably best matches the situation in the
sense that lockdep would actually be able to know about the nesting
going on.

> Another idea is to have A's method temporarily drop the sysfs readlock.  
> Of course that would put the onus on the USB core of guaranteeing that
> A cannot be removed while this happens, but we can handle that.

Yeah, that's an easier way out.  Please make it a proper sysfs API
call tho so that people working on sysfs later can know of the special
case.

Thanks.

-- 
tejun
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