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); > > > > > > -- > > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > > the body of a message to majord...@vger.kernel.org > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Please read the FAQ at http://www.tux.org/lkml/ > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/