linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Date:	Mon, 10 Jun 2013 17:08:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Sun, 2013-06-09 at 12:36 -0700, Paul E. McKenney wrote:
>  
> +#else /* #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +
> +bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc);
> +
> +static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
> +{
> +	register struct __raw_tickets inc = { .tail = 2 };
> +
> +	inc = xadd(&lock->tickets, inc);
> +	for (;;) {
> +		if (inc.head == inc.tail || tkt_spin_pass(lock, inc))
> +			break;
> +		inc.head = ACCESS_ONCE(lock->tickets.head);
> +	}
> +	barrier(); /* smp_mb() on Power or ARM. */
> +}
> +
> +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +

To avoid the above code duplication, I would have this instead:

#ifdef CONFIG_TICKET_LOCK_QUEUED

bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc);
#define __TKT_SPIN_INC 2

#else

static inline bool tkt_spin_pass(arch_spinlock_t *ap, struct
__raw_tickets inc)
{
	return false;
}

#define __TKT_SPIN_INC 1

#endif

static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
{
	register struct __raw_tickets inc = { .tail = __TKT_SPIN_INC };

	inc = xadd(&lock->tickets, inc);

	for (;;) {
		if (inc.head == inc.tail || tkt_spin_pass(lock, inc))
			break;
		cpu_relax();
		inc.head = ACCESS_ONCE(lock->tickets.head);
	}
	barrier;	/* make sure nothing creeps before the lock is taken */
}

-- Steve


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