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

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Date:	Fri, 13 Feb 2015 14:49:24 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Jiri Slaby <jslaby@...e.cz>, Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 8/9] livepatch: allow patch modules to be removed

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.

-- 
Josh
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