Messages in this thread | | | Date | Thu, 16 Apr 2009 20:41:58 +0200 | From | Eric Dumazet <> | Subject | Re: [PATCH] netfilter: use per-cpu recursive spinlock (v6) |
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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
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