Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756022Ab3FLLGz (ORCPT ); Wed, 12 Jun 2013 07:06:55 -0400 Received: from mail-ie0-f176.google.com ([209.85.223.176]:54537 "EHLO mail-ie0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753106Ab3FLLGy (ORCPT ); Wed, 12 Jun 2013 07:06:54 -0400 MIME-Version: 1.0 In-Reply-To: <1371002288.9844.246.camel@gandalf.local.home> References: <20130609193657.GA13392@linux.vnet.ibm.com> <51B748DA.2070306@hp.com> <20130611163607.GG5146@linux.vnet.ibm.com> <51B76F77.6020004@hp.com> <20130611194927.GX5146@linux.vnet.ibm.com> <1371002288.9844.246.camel@gandalf.local.home> Date: Wed, 12 Jun 2013 19:06:53 +0800 Message-ID: Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock From: Lai Jiangshan To: Steven Rostedt Cc: paulmck@linux.vnet.ibm.com, Waiman Long , linux-kernel@vger.kernel.org, mingo@elte.hu, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, torvalds@linux-foundation.org, Davidlohr Bueso Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 12515 Lines: 308 On Wed, Jun 12, 2013 at 9:58 AM, 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. No, you are right. When I lay on the bed in the night, I was thinking about the V1, I wrongly considered the V2 has the same problem without deeper thought in this morning. V2 has not such problem. sorry for the noisy. Thanks, Lai > > -- 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 majordomo@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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/