Re: [RFC PATCH 8/9] livepatch: allow patch modules to be removed

From: Josh Poimboeuf
Date: Fri Feb 13 2015 - 16:01:00 EST

On Fri, Feb 13, 2015 at 05:17:10PM +0100, Miroslav Benes wrote:
> On Fri, 13 Feb 2015, Josh Poimboeuf wrote:
> > Hm, even with Jiri Slaby's suggested fix to add the completion to the
> > unregister path, I still get a lockdep warning. This looks more insidious,
> > related to the locking order of a kernfs lock and the klp lock. I'll need to
> > look at this some more...
> Yes, I was afraid of this. Lockdep warning is a separate bug. It is caused
> by taking klp_mutex in enabled_store. During rmmod klp_unregister_patch
> takes klp_mutex and destroys the sysfs structure. If somebody writes to
> enabled just after unregister takes the mutex and before the sysfs
> removal, he would cause the deadlock, because enabled_store takes the
> "sysfs lock" and then klp_mutex. That is exactly what the lockdep tells us
> below.
> We can look for inspiration elsewhere. Grep for s_active through git log
> of the mainline offers several commits which dealt exactly with this. Will
> browse through that...

Thanks Miroslav, please let me know what you find. It wouldn't surprise
me if this were a very common problem.

One option would be to move the enabled_store() work out to a workqueue
or something.

> >
> > To recreate:
> >
> > insmod livepatch-sample.ko
> >
> > # wait for patching to complete
> >
> > ~/a.out & <-- simple program which opens the "enabled" file in the background
> I didn't even need such a program. Lockdep warned me with sole insmod,
> echo and rmmod. It is magically clever.

Ah, even easier... lockdep is awesome.

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