linux-kernel - Re: [PATCH] [RFC] EEE PC hangs when booting off battery

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Date:	Tue, 28 Apr 2009 11:58:08 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Alan Jenkins <alan-jenkins@...fmail.co.uk>
Cc:	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [PATCH] [RFC] EEE PC hangs when booting off battery

It seems the only reason lockdep doesn't warn is the detour to userspace
(modprobe) and the waiting for a userspace task (waiting isn't lockdep
annotated and I don't think it can be)

It seems you cannot load a module while under _any_ lock, ever, since
userspace might turn around and do something that requires that lock? I
think we should probably add a warning to the code that waits for the
userspace task:

	debug_check_no_locks_held(current);

Or not? Some purely internal locks _might_ be ok... but how could you
verify that?

> - ieee80211_wep_init(), which is called with rtnl_lock() held, is
> blocked in request_module() [waiting for modprobe to load a crypto module].

Right.

> - modprobe is blocked in a call to flush_workqueue(), caused by closing
> a TTY.

Can you point out where? I can't find that.

> - worker_thread is blocked because the workqueue item linkwatch_event()
> is blocked on rtnl_lock.

This I know about.

> I've hacked up a test patch to move wep_init() outside of rtnl_lock, and
> it solved the problem.  My one caveat is that it would probably be
> cleaner to move it after rtnl_unlock(), instead of before rtnl_lock(). 
> I just wasn't 100% sure if that would be safe.  Here's the patch:

That doesn't seem relevant, this just does some initialisation. However,
you definitely missed adding a call to wep_free().

johannes

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