Re: [PATCH RFC ticketlock] Auto-queued ticketlock

[Steven Rostedt]

On Tue, 2013-06-11 at 12:49 -0700, Paul E. McKenney wrote:

> +bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc)
> +{
> +	if (unlikely(inc.head & 0x1)) {
> +
> +		/* This lock has a queue, so go spin on the queue. */
> +		if (tkt_q_do_spin(ap, inc))
> +			return true;
> +
> +		/* Get here if the queue is in transition: Retry next time. */
> +

This looks better, but please add a comment, something to the likes of:

	/*
	 * Only the TKT_Q_SWITCH waiter will set up the queue to prevent
	 * a thundering herd of setups to occur. It is still possible for
	 * more than one task to perform a setup if the lock is released
	 * after this check, a waiter coming in may also match this test. But
	 * that's covered by the cmpxchg() setup in tkt_q_start_contend.
	 */


> +	} else if (inc.tail - TKT_Q_SWITCH == inc.head) {

Also shouldn't this be:

	} else if ((__ticket_t)(inc.tail - TKT_Q_SWITCH) == inc.head) {

As TKT_Q_SWITCH doesn't have a type, I'm not sure how C will evaluate
this. I always screw type conversions up, and just add in the type casts
to be safe.

You could also give TKT_Q_SWITCH a type too.

-- Steve

> +
> +		/*
> +		 * This lock has lots of spinners, but no queue.
> +		 * Go create a queue to spin on.
> +		 */
> +		if (tkt_q_start_contend(ap, inc))
> +			return true;
> +
> +		/* Get here if the queue is in transition: Retry next time. */
> +	}
> +
> +	/* Either no need for a queue or the queue is in transition.  Spin. */
> +	return false;
> +}
> +EXPORT_SYMBOL(tkt_spin_pass);


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