Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

On Fri, Feb 07, 2014 at 04:18:47PM +0100, Peter Zijlstra wrote:

void ticket_lock(tickets_t *lock)
{
	tickets_t t;

	/*
	 * Because @head is MSB, the direct increment wrap doesn't disturb
	 * @tail.
	 */
	t.pair = xadd(&lock->pair, 1<<16);

	if (likely(t.head == t.tail)) {
		__lwsync(); /* acquire */
		return;
	}

	while (smp_load_acquire(&lock->tail) != t.tail)
		cpu_relax();

That should be "!= t.head", for that contains our ticket.

I'm hopelessly scatter brained today it seems :/

}

void ticket_unlock(tickets_t *lock)
{
	ticket_t tail = lock->tail + 1;

	/*
	 * The store is save against the xadd for it will make the ll/sc fail
	 * and try again. Aside from that PowerISA guarantees single-copy
	 * atomicy for half-word writes.
	 *
	 * And since only the lock owner will ever write the tail, we're good.
	 */
	smp_store_release(&lock->tail, tail);
}