Re: [PATCH RFC ticketlock] Auto-queued ticketlock

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 11, 2013 at 09:58:08PM -0400, Steven Rostedt wrote:
> On Wed, 2013-06-12 at 09:19 +0800, Lai Jiangshan wrote:
> 
> > > +
> > > +/*
> > > + * Hand the lock off to the first CPU on the queue.
> > > + */
> > > +void tkt_q_do_wake(arch_spinlock_t *lock)
> > > +{
> > > +       struct tkt_q_head *tqhp;
> > > +       struct tkt_q *tqp;
> > > +
> > > +       /* If the queue is still being set up, wait for it. */
> > > +       while ((tqhp = tkt_q_find_head(lock)) == NULL)
> > > +               cpu_relax();
> > > +
> > > +       for (;;) {
> > > +
> > > +               /* Find the first queue element. */
> > > +               tqp = ACCESS_ONCE(tqhp->spin);
> > > +               if (tqp != NULL)
> > > +                       break;  /* Element exists, hand off lock. */
> > > +               if (tkt_q_try_unqueue(lock, tqhp))
> > > +                       return; /* No element, successfully removed queue. */
> > > +               cpu_relax();
> > > +       }
> > > +       if (ACCESS_ONCE(tqhp->head_tkt) != -1)
> > > +               ACCESS_ONCE(tqhp->head_tkt) = -1;
> > > +       smp_mb(); /* Order pointer fetch and assignment against handoff. */
> > > +       ACCESS_ONCE(tqp->cpu) = -1;
> > > +}
> > > +EXPORT_SYMBOL(tkt_q_do_wake);
> > > +
> > > +/*
> > > + * Given a lock that already has a queue associated with it, spin on
> > > + * that queue.  Return false if there was no queue (which means we do not
> > > + * hold the lock) and true otherwise (meaning we -do- hold the lock).
> > > + */
> > > +bool tkt_q_do_spin(arch_spinlock_t *lock, struct __raw_tickets inc)
> > > +{
> > > +       struct tkt_q **oldtail;
> > > +       struct tkt_q tq;
> > > +       struct tkt_q_head *tqhp;
> > > +
> > > +       /*
> > > +        * Ensure that accesses to queue header happen after sensing
> > > +        * the lock's have-queue bit.
> > > +        */
> > > +       smp_mb();  /* See above block comment. */
> > > +
> > > +       /* If there no longer is a queue, leave. */
> > > +       tqhp = tkt_q_find_head(lock);
> > > +       if (tqhp == NULL)
> > > +               return false;
> > > +
> > > +       /* Initialize our queue element. */
> > > +       tq.cpu = raw_smp_processor_id();
> > > +       tq.tail = inc.tail;
> > > +       tq.next = NULL;
> > 
> > I guess a mb() is needed here for between read tqhp->ref and read
> > tqhp->head_tkt.
> > you can move the above mb() to here.
> 
> Do we?
> 
> The only way to get into here is if you either set up the queue
> yourself, or you saw the LSB set in head.
> 
> If you were the one to set it up yourself, then there's nothing to worry
> about because you are also the one that set head_tkt.
> 
> If you didn't set up the queue, then someone else set the LSB in head,
> which is done with a cmpxchg() which is also a full mb. This would make
> head_tkt visible as well because it's set before cmpxchg is called.
> 
> Thus, to come into this function you must have seen head & 1 set, and
> the smp_mb() above will also make head_tkt visible.
> 
> The only thing I can see now is that it might not find tqhp because ref
> may not be set yet. If that's the case, then it will fall out back to
> the main loop. But if it finds ref, then I don't see how it can't see
> head_tkt up to date as well.
> 
> Maybe I'm missing something.

Hmmm...  I need to look at this more carefully.  Lai might well be right
because if we are relying on the cmpxchg() for ordering, there needs
to be a memory barrier on the read side to pair with the cmpxchg().
You are of course quite correct in the case where the CPU reading the
->head_tkt is the one that set it up.

Something to think about at the gym, I guess.  ;-)

							Thanx, Paul

> -- Steve
> 
> 
> > 
> > > +
> > > +       /* Check to see if we already hold the lock. */
> > > +       if (ACCESS_ONCE(tqhp->head_tkt) == inc.tail) {
> > > +               /* The last holder left before queue formed, we hold lock. */
> > > +               tqhp->head_tkt = -1;
> > > +               return true;
> > > +       }
> > > +
> > > +       /*
> > > +        * Add our element to the tail of the queue.  Note that if the
> > > +        * queue is empty, the ->spin_tail pointer will reference
> > > +        * the queue's head pointer, namely ->spin.
> > > +        */
> > > +       oldtail = xchg(&tqhp->spin_tail, &tq.next);
> > > +       ACCESS_ONCE(*oldtail) = &tq;
> > > +
> > > +       /* Spin until handoff. */
> > > +       while (ACCESS_ONCE(tq.cpu) != -1)
> > > +               cpu_relax();
> > > +
> > > +       /*
> > > +        * Remove our element from the queue.  If the queue is now empty,
> > > +        * update carefully so that the next acquisition will enqueue itself
> > > +        * at the head of the list.  Of course, the next enqueue operation
> > > +        * might be happening concurrently, and this code needs to handle all
> > > +        * of the possible combinations, keeping in mind that the enqueue
> > > +        * operation happens in two stages: (1) update the tail pointer and
> > > +        * (2) update the predecessor's ->next pointer.  With this in mind,
> > > +        * the following code needs to deal with three scenarios:
> > > +        *
> > > +        * 1.   tq is the last entry.  In this case, we use cmpxchg to
> > > +        *      point the list tail back to the list head (->spin).  If
> > > +        *      the cmpxchg fails, that indicates that we are instead
> > > +        *      in scenario 2 below.  If the cmpxchg succeeds, the next
> > > +        *      enqueue operation's tail-pointer exchange will enqueue
> > > +        *      the next element at the queue head, because the ->spin_tail
> > > +        *      pointer now references the queue head.
> > > +        *
> > > +        * 2.   tq is the last entry, and the next entry has updated the
> > > +        *      tail pointer but has not yet updated tq.next.  In this
> > > +        *      case, tq.next is NULL, the cmpxchg will fail, and the
> > > +        *      code will wait for the enqueue to complete before completing
> > > +        *      removal of tq from the list.
> > > +        *
> > > +        * 3.   tq is not the last pointer.  In this case, tq.next is non-NULL,
> > > +        *      so the following code simply removes tq from the list.
> > > +        */
> > > +       if (tq.next == NULL) {
> > > +
> > > +               /* Mark the queue empty. */
> > > +               tqhp->spin = NULL;
> > > +
> > > +               /* Try to point the tail back at the head. */
> > > +               if (cmpxchg(&tqhp->spin_tail,
> > > +                           &tq.next,
> > > +                           &tqhp->spin) == &tq.next)
> > > +                       return true; /* Succeeded, queue is now empty. */
> > > +
> > > +               /* Failed, if needed, wait for the enqueue to complete. */
> > > +               while (tq.next == NULL)
> > > +                       cpu_relax();
> > > +
> > > +               /* The following code will repair the head. */
> > > +       }
> > > +       smp_mb(); /* Force ordering between handoff and critical section. */
> > > +
> > > +       /*
> > > +        * Advance list-head pointer.  This same task will be the next to
> > > +        * access this when releasing the lock, so no need for a memory
> > > +        * barrier after the following assignment.
> > > +        */
> > > +       ACCESS_ONCE(tqhp->spin) = tq.next;
> > > +       return true;
> > > +}
> > > +
> > > +/*
> > > + * Given a lock that does not have a queue, attempt to associate the
> > > + * i-th queue with it, returning true if successful (meaning we hold
> > > + * the lock) or false otherwise (meaning we do -not- hold the lock).
> > > + * Note that the caller has already filled in ->ref with 0x1, so we
> > > + * own the queue.
> > > + */
> > > +static bool
> > > +tkt_q_init_contend(int i, arch_spinlock_t *lock, struct __raw_tickets inc)
> > > +{
> > > +       arch_spinlock_t asold;
> > > +       arch_spinlock_t asnew;
> > > +       struct tkt_q_head *tqhp;
> > > +
> > > +       /* Initialize the i-th queue header. */
> > > +       tqhp = &tkt_q_heads[i];
> > > +       tqhp->spin = NULL;
> > > +       tqhp->spin_tail = &tqhp->spin;
> > > +
> > > +       /* Each pass through this loop attempts to mark the lock as queued. */
> > > +       do {
> > > +               asold.head_tail = ACCESS_ONCE(lock->head_tail);
> > > +               asnew = asold;
> > > +               if (asnew.tickets.head & 0x1) {
> > > +
> > > +                       /* Someone beat us to it, back out. */
> > > +                       smp_mb();
> > > +                       ACCESS_ONCE(tqhp->ref) = NULL;
> > > +
> > > +                       /* Spin on the queue element they set up. */
> > > +                       return tkt_q_do_spin(lock, inc);
> > > +               }
> > > +
> > > +               /*
> > > +                * Record the head counter in case one of the spinning
> > > +                * CPUs already holds the lock but doesn't realize it yet.
> > > +                */
> > > +               tqhp->head_tkt = asold.tickets.head;
> > > +
> > > +               /* The low-order bit in the head counter says "queued". */
> > > +               asnew.tickets.head |= 0x1;
> > > +       } while (cmpxchg(&lock->head_tail,
> > > +                asold.head_tail,
> > > +                asnew.head_tail) != asold.head_tail);
> > > +
> > > +       /* Point the queue at the lock and go spin on it. */
> > > +       ACCESS_ONCE(tqhp->ref) = lock;
> > > +       return tkt_q_do_spin(lock, inc);
> > > +}
> > > +
> > > +/*
> > > + * Start handling a period of high contention by finding a queue to associate
> > > + * with this lock.  Returns true if successful (in which case we hold the
> > > + * lock) and false otherwise (in which case we do -not- hold the lock).
> > > + */
> > > +bool tkt_q_start_contend(arch_spinlock_t *lock, struct __raw_tickets inc)
> > > +{
> > > +       int i;
> > > +       int start;
> > > +
> > > +       /* Hash the lock address to find a starting point. */
> > > +       start = i = tkt_q_hash(lock);
> > > +
> > > +       /*
> > > +        * Each pass through the following loop attempts to associate
> > > +        * the lock with the corresponding queue.
> > > +        */
> > > +       do {
> > > +               /*
> > > +                * Use 0x1 to mark the queue in use, but also avoiding
> > > +                * any spinners trying to use it before we get it all
> > > +                * initialized.
> > > +                */
> > > +               if (tkt_q_heads[i].ref)
> > > +                       continue;
> > > +               if (cmpxchg(&tkt_q_heads[i].ref,
> > > +                           NULL,
> > > +                           (arch_spinlock_t *)0x1) == NULL) {
> > > +
> > > +                       /* Succeeded, now go initialize it. */
> > > +                       return tkt_q_init_contend(i, lock, inc);
> > > +               }
> > > +
> > > +               /* If someone beat us to it, go spin on their queue. */
> > > +               if (ACCESS_ONCE(lock->tickets.head) & 0x1)
> > > +                       return tkt_q_do_spin(lock, inc);
> > > +       } while ((i = tkt_q_next_slot(i)) != start);
> > > +
> > > +       /* All the queues are in use, revert to spinning on the ticket lock. */
> > > +       return false;
> > > +}
> > > +
> > > +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. */
> > > +
> > > +       } else if (inc.tail - TKT_Q_SWITCH == inc.head) {
> > > +
> > > +               /*
> > > +                * 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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