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);
> }
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