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

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Date:	Thu, 11 Feb 2010 14:47:57 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Greg KH <gregkh@...e.de>
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

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)
                                     ^^^^

Eric



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