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SubjectRe: [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@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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