linux-kernel - Re: [PATCH] sysfs: differentiate between locking links and non-links

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Date:	Thu, 18 Feb 2010 11:39:12 +1100
From:	Neil Brown <neilb@...e.de>
To:	Greg KH <greg@...ah.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg KH <gregkh@...e.de>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysfs: differentiate  between locking links and
 non-links

On Wed, 17 Feb 2010 14:38:48 -0800
Greg KH <greg@...ah.com> wrote:

> On Thu, Feb 11, 2010 at 02:47:57PM -0800, Eric W. Biederman wrote:
> > Greg KH <gregkh@...e.de> writes:
> > 
> > > 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?
> > 
> >  
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > -#define sysfs_dirent_init_lockdep(sd)				\
> > +#define sysfs_dirent_init_lockdep(sd, type)			\
> >  do {								\
> >  	static struct lock_class_key __key;			\
> >  								\
> > -	lockdep_init_map(&sd->dep_map, "s_active", &__key, 0);	\
> > +	lockdep_init_map(&sd->dep_map, "s_active_" type, &__key, 0);	\
> >  } while(0)
> >  #else
> >  #define sysfs_dirent_init_lockdep(sd) do {} while(0)
> 
> Got it, I've fixed this by hand.
> 
>

Thanks.
I hadn't sent you a fix myself (As requested) as I got the impression from
the following discussion that a different approach would be taken.

I'm happy either way though.

Thanks,
NeilBrown
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