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

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Message-ID: <alpine.LNX.2.00.1502161657380.9435@pobox.suse.cz>
Date:	Mon, 16 Feb 2015 17:06:15 +0100 (CET)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
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, 13 Feb 2015, Josh Poimboeuf wrote:

> 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.

Yes, that is one possibility. It is not the only one.

1. we could replace mutex_lock in enabled_store with mutex_trylock. If the 
lock was not acquired we would return -EBUSY. Or could we 'return 
restart_syscall' (maybe after some tiny msleep)?

2. we could reorganize klp_unregister_patch somehow and move sysfs removal 
out of mutex protection.

Miroslav

> > > 
> > > 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
> 

--
Miroslav Benes
SUSE Labs
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