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

From: Eric Dumazet <[email-masked]>
Date: Tue, 21 Apr 2009 21:46:27 +0200

Ingo Molnar a écrit :
> 
> Why not use the obvious solution: a _single_ wrlock for global 
> access and read_can_lock() plus per cpu locks in the fastpath?

Obvious is not the qualifier I would use :)

Brilliant yes :)

> 
> That way there's no global cacheline bouncing (just the _reading_ of 
> a global cacheline - which will be nicely localized - on NUMA too) - 
> and we will hold at most 1-2 locks at once!
> 
> Something like:
> 
> 	__cacheline_aligned DEFINE_RWLOCK(global_wrlock);
> 
> 	DEFINE_PER_CPU(rwlock_t local_lock);
> 
> 
> 	void local_read_lock(void)
> 	{
> 	again:
> 		read_lock(&per_cpu(local_lock, this_cpu));

Hmm... here we can see global_wrlock locked by on writer, while
this cpu already called local_read_lock(), and calls again this
function -> Deadlock, because we hold our local_lock locked.

> 
> 		if (unlikely(!read_can_lock(&global_wrlock))) {
> 			read_unlock(&per_cpu(local_lock, this_cpu));
> 			/*
> 			 * Just wait for any global write activity:
> 			 */
> 			read_unlock_wait(&global_wrlock);
> 			goto again;
> 		}
> 	}
> 
> 	void global_write_lock(void)
> 	{
> 		write_lock(&global_wrlock);
> 
> 		for_each_possible_cpu(i)
> 			write_unlock_wait(&per_cpu(local_lock, i));
> 	}
> 
> Note how nesting friendly this construct is: we dont actually _hold_ 
> NR_CPUS locks all at once, we simply cycle through all CPUs and make 
> sure they have our attention.
> 
> No preempt overflow. No lockdep explosion. A very fast and scalable 
> read path.
> 
> Okay - we need to implement read_unlock_wait() and 
> write_unlock_wait() which is similar to spin_unlock_wait(). The 
> trivial first-approximation is:
> 
> 	read_unlock_wait(x)
> 	{
> 		read_lock(x);
> 		read_unlock(x);
> 	}
> 
> 	write_unlock_wait(x)
> 	{
> 		write_lock(x);
> 		write_unlock(x);
> 	}
> 

Very interesting and could be changed to use spinlock + depth per cpu.

-> we can detect recursion and avoid the deadlock, and we only use one
atomic operation per lock/unlock pair in fastpath (this was the reason we
tried hard to use a percpu spinlock during this thread)

__cacheline_aligned DEFINE_RWLOCK(global_wrlock);

struct ingo_local_lock {
	spinlock_t lock;
	int depth;
};
DEFINE_PER_CPU(struct ingo_local_lock local_lock);

void local_read_lock(void)
{
	struct ingo_local_lock *lck;

	local_bh_and_preempt_disable();
	lck = &get_cpu_var(local_lock);
	if (++lck->depth > 0) /* already locked */
		return;
again:
	spin_lock(&lck->lock);

	if (unlikely(!read_can_lock(&global_wrlock))) {
		spin_unlock(&lck->lock);
		/*
		 * Just wait for any global write activity:
		 */
		read_unlock_wait(&global_wrlock);
		goto again;
	}
}

void global_write_lock(void)
{
	write_lock(&global_wrlock);

	for_each_possible_cpu(i)
		spin_unlock_wait(&per_cpu(local_lock, i));
}

Hmm ?

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