Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v11)

From: Ingo Molnar <[email-masked]>
Date: Tue, 21 Apr 2009 20:02:25 +0200

* Linus Torvalds <[email-masked]> wrote:

> PS: Ingo, why do the *_bh() functions in kernel/spinlock.c do 
> _both_ a "local_bh_disable()" and a "preempt_disable()"? BH 
> disable should disable preemption too, no? Or am I confused? In 
> which case we need that in the above rdlock_bh too.

i think there might be (are?) uses of:

	spin_lock_bh(&some->lock);
	...
	spin_unlock(&some->lock);
	...
	local_bh_enable();

So we have to have two preemption control levels for that, as 
there's no knowledge at the spin_lock_bh() place whether it will be 
followed by a spin_unlock_bh() [in which case it would be safe to do 
SOFTIRQ_OFFSET only] - or by a spin_unlock() + local_bh_enable() 
pair..

[ That locking pattrn even makes a certain amount of sense: keep the 
  lock held for a short amount of time - then weaken locking to bh 
  context exclusion only. ]

What we could do is an optimization to do a compound increase the 
preempt count by SOFTIRQ_OFFSET+1 - instead of a local_bh_disable() 
+ preempt_disable()? Symmetrically we could do a compound decrease 
in the unlock case.

It might even be called: local_bh_preempt_disable() or so?

	Ingo