Re: [PATCH 3/4] net/netfilter/nf_conntrack_core: update memory barriers.

Peter Zijlstra
2016-08-29T10:51:32Z

On Sun, Aug 28, 2016 at 01:56:15PM +0200, Manfred Spraul wrote:
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -80,20 +80,29 @@ static __read_mostly bool nf_conntrack_locks_all;
>
> void nf_conntrack_lock(spinlock_t *lock) __acquires(lock)
> {
> + /* Step 1: Acquire the lock */
> spin_lock(lock);
> - while (unlikely(nf_conntrack_locks_all)) {
> - spin_unlock(lock);
>
> - /*
> - * Order the 'nf_conntrack_locks_all' load vs. the
> - * spin_unlock_wait() loads below, to ensure
> - * that 'nf_conntrack_locks_all_lock' is indeed held:
> - */
> - smp_rmb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
> - spin_unlock_wait(&nf_conntrack_locks_all_lock);
> - spin_lock(lock);
> - }
> + /* Step 2: make it visible to all CPUs that we hold the lock */
> + smp_mb__after_spin_lock();

I hate this comment. A barrier does _not_ make visible anything.

A barrier _orders_ things.