Messages in this thread | | | Date | Tue, 21 Apr 2009 20:02:25 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] netfilter: use per-cpu recursive lock (v11) |
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* 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
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