Messages in this thread | | | Date | Thu, 11 Feb 2010 14:32:35 -0800 | From | Greg KH <> | Subject | Re: [PATCH] sysfs: differentiate between locking links and non-links |
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On Thu, Feb 11, 2010 at 01:42:10PM -0800, Eric W. Biederman wrote: > Greg KH <gregkh@suse.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@suse.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@suse.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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