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

Ingo Molnar
2009-04-21T18:03:26Z

* Linus Torvalds <torvalds@linux-foundation.org> 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