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

From: Peter Zijlstra <[email-masked]>
Date: Fri, 7 Feb 2014 16:43:46 +0100

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);
> }