linux-kernel - Re: [PATCH/RFC] remove irqs_disabled warning from local_bh

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Date:	Tue, 17 Jun 2008 16:55:02 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Johannes Berg <johannes@...solutions.net>
cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Michael Buesch <mb@...sch.de>,
	David Ellingsworth <david@...ntd.dyndns.org>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH/RFC] remove irqs_disabled warning from local_bh_enable



On Tue, 17 Jun 2008, Johannes Berg wrote:
>
> This warning has started to trigger with mac80211 because it can, under
> some circumstances, use spin_lock_bh() protected sections within
> irq-disabled sections. Is that a bug?

Yes, it's a bug.

Why? Not because of the "spin_lock_bh()" itself, but because of the 
_unlock_, which does a "local_bh_enable_ip()", which in turn will check 
the whole "do_softirq()" if it was the last softirq_count.

And you must not do softirq's when hard-irq's were disabled!

So it should in theory be ok (but perhaps a bit odd) to do something like

	spin_lock_irq(&irq_lock);
	..do something..
	spin_lock_bh(&bh_lock);
	spin_unlock_irq(&irq_lock);
	.. do something else ..
	spin_unlock_bh(&bh_lock);

where the "spin_lock_bh()" itself is in an irq-locked context - as long as 
the "spin_unlock_bh()" is *not*.

See?

		Linus
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