Re: [PATCH] netfilter: use per-cpu recursive spinlock (v6)

Eric Dumazet
2009-04-16T18:42:41Z

Paul E. McKenney a écrit :
>
> But if some other CPU holds the lock, this code would fail to wait for
> that other CPU to release the lock, right? It also might corrupt the
> rl->count field due to two CPUs accessing it concurrently non-atomically.

If another cpu holds the lock, this cpu will spin on its own lock.

Remember other cpus dont touch rl->count. This is a private field, only touched
by the cpu on its own per_cpu data. There is no possible 'corruption'

So the owner of the per_cpu data does :

/*
* disable preemption, get rl = &__get_cpu_var(arp_tables_lock);
* then :
*/
lock_time :
if (++rl->count == 0)
spin_lock(&rl->lock);

unlock_time:
if (likely(--rl->count == 0))
spin_unlock(&rl->lock);

while other cpus only do :

spin_lock(&rl->lock);
/* work on data */
spin_unlock(&rl->lock);

So they cannot corrupt 'count' stuff.

>
> I suggest the following, preferably in a function or macro or something:
>
> cur_cpu = smp_processor_id();
> if (likely(rl->owner != cur_cpu) {
> spin_lock(&rl->lock);
> rl->owner = smp_processor_id();
> rl->count = 1;
> } else {
> rl->count++;
> }
>
> And the inverse for unlock.
>
> Or am I missing something subtle?

Apparently Linus missed it too, and reacted badly to my mail.
I dont know why we discuss of this stuff on lkml either...

I stop working on this subject and consider drinking dome hard stuf and
watching tv :)

See you