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SubjectRe: [PATCH] sysfs: differentiate between locking links and non-links
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On Wednesday 10 February 2010 09:36:32 am Eric W. Biederman wrote:
> Neil Brown <neilb@suse.de> writes:
> > 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).
>
> You are correct. Not until the file handles are closed but until all
> users of the underyling methods are complete.
>
> > 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 for this.
>
> Separating out symlinks and treating them differently because they can not
> cause problems is definitely worth doing. We never take an active
> reference in the symlink code so we can never block waiting for symlinks
> to be deleted.
>
>
> We block when deleting files in sysfs (and proc and sysctl). If we
> did not block we could follow pointers into modules that are being
> deleted, or those methods that are running could access data
> structures that we want to tear down (perhaps there is a lock we want
> to kfree). Blocking in sysfs is to simplify the life of the callers.
> Unfortunately for a handful of callers it complicates things.

Exactly. Before Tejun changed sysfs to provide guarantee that no
show/store methods are still running, nor new references to the
corresponding kobject will be acquired through sysfs after
sysfs_remove_file() returns, you had to jump through million of
hoops at subsystem level to work with lifetime rules and work around
the fact that kobjects could outlive your module.

I was glad to see bunch of ugly code in serio, gameport and input go
and I do not want it coming back ;)

>
> If you want to compare this to regular files think of what sysfs is
> doing as a combined remove and revoke. The remove is easy. Revoke
> is just plane difficult.
>
> Eric

--
Dmitry


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