linux-kernel - Recursive deactivation of local bottom halves

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Date:	Tue, 25 Jan 2011 14:05:14 +0100
From:	Emil Langrock <emil.langrock@....de>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Recursive deactivation of local bottom halves

Hi,

I wanted or more or less had to do following:

...
spin_lock_bh(lock1);
....
	spin_lock_bh(lock2);
	...
	spin_unlock_bh(lock2);
....
spin_unlock_bh(lock1);
....

Now some weird behavior were noticed by a second person and he fixed it by 
removing the _bh from the inner spinlocks. I checked what the difference is 
and here is the previous example without the common spin_lock behaviour:

...
local_bh_disable();
....
	local_bh_disable();
	...
	local_bh_enable();
....
local_bh_enable();
....

According to http://people.netfilter.org/rusty/unreliable-guides/kernel-
hacking/routines-softirqs.html it makes no difference how often I use the pair 
local_bh_disable/local_bh_enable inside local_bh_disable/local_bh_enable. So 
was this changed?

I also couldn't find any indication in the kernel/softirq.c in the 
local_bh_enable functionality that the counting in local_bh_disable makes any 
difference.

Can somebody tell me if that is legal or not (on smp and uniprocessor)? And 
maybe can show me the related source code why is it may or may not be legal?

Kind regards,
Emil
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