Message-ID: <20100211223235.GC30430@suse.de>
Date:	Thu, 11 Feb 2010 14:32:35 -0800
From:	Greg KH <gregkh@...e.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Neil Brown <neilb@...e.de>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysfs: differentiate  between locking links and
 non-links

On Thu, Feb 11, 2010 at 01:42:10PM -0800, Eric W. Biederman wrote:
> Greg KH <gregkh@...e.de> writes:
> 
> > On Wed, Feb 10, 2010 at 12:09:33PM +1100, Neil Brown wrote:
> >> 
> >> Hi,
> >>  I've just spent a while sorting out some lockdep complaints triggered
> >>  by the recent addition of the "s_active" lockdep annotation in sysfs
> >>   (commit 846f99749ab68bbc7f75c74fec305de675b1a1bf)
> >> 
> >>  Some of them are genuine and I have submitted a fix for those.
> >>  Some are, I think, debatable and I get to that is a minute.  I've
> >>  submitted a fix for them anyway.
> >>  But some are to my mind clearly bogus and I'm hoping that can be
> >>  fixed by the change below (or similar).
> >>  The 'bogus' ones are triggered by writing to a sysfs attribute file
> >>  for which the handler tries to delete a symlink from sysfs.
> >>  This appears to be a recursion on s_active as s_active is held while
> >>  the handler runs and is again needed to effect the delete.  However
> >>  as the thing being deleted is a symlink, it is very clearly a
> >>  different object to the thing triggering the delete, so there is no
> >>  real loop.
> >> 
> >>  The following patch splits the lockdep context in two - one for
> >>  symlink and one for everything else.  This removes the apparent loop.
> >>  (An example report can be seen in
> >>      http://bugzilla.kernel.org/show_bug.cgi?id=15142).
> >> 
> >>  The "debatable" dependency loops happen when writing to one attribute
> >>  causes a different attribute to be deleted.  In my (md) case this can
> >>  actually cause a deadlock as both the attributes take the same lock
> >>  while the handler is running.  This is because deleting the attribute
> >>  will block until the all accesses of that attribute have completed (I
> >>  think).
> >>  However it should be possible to delete a name from sysfs while there
> >>  are still accesses pending (it works for normal files!!).  So if
> >>  sysfs could be changed to simply unlink the file and leave deletion to
> >>  happen when the refcount become zero it would certainly make my life
> >>  a lot easier, and allow the removal of some ugly code from md.c.
> >>  I don't know sysfs well enough to suggest a patch though.
> >> 
> >> Thanks,
> >> NeilBrown
> >> 
> >> 
> >> 
> >> commit 2e502cfe444b68f6ef6b8b2abe83b6112564095b
> >> Author: NeilBrown <neilb@...e.de>
> >> Date:   Wed Feb 10 09:43:45 2010 +1100
> >> 
> >>     sysfs: differentiate  between locking links and non-links for sysfs
> >>     
> >>     symlinks and non-symlink is sysfs are very different.
> >>     A symlink can never be locked (active) while an attribute
> >>     modification routine is running.  So removing symlink from an
> >>     attribute 'store' routine should be permitted without any lockdep
> >>     warnings.
> >>     
> >>     So split the lockdep context for 's_active' in two, one for symlinks
> >>     and other for everything else.
> >>     
> >>     Signed-off-by: NeilBrown <neilb@...e.de>
> >
> > Nice patch, I'll queue it up for .34.
> 
> Note the patch does not compile with lockdep disabled.

Ugh, why not?

Neil, care to fix this up?

thanks,

greg k-h
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