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Date: Tue, 11 Jun 2013 23:10:30 +0800
From: Lai Jiangshan <eag0628@...il.com>
To: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
josh@...htriplett.org, niv@...ibm.com,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
David Howells <dhowells@...hat.com>,
Eric Dumazet <edumazet@...gle.com>, darren@...art.com,
Frédéric Weisbecker <fweisbec@...il.com>,
Silas Boyd-Wickizer <sbw@....edu>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock
On Tue, Jun 11, 2013 at 10:48 PM, Lai Jiangshan <eag0628@...il.com> wrote:
> On Mon, Jun 10, 2013 at 3:36 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
>> Breaking up locks is better than implementing high-contention locks, but
>> if we must have high-contention locks, why not make them automatically
>> switch between light-weight ticket locks at low contention and queued
>> locks at high contention?
>>
>> This commit therefore allows ticket locks to automatically switch between
>> pure ticketlock and queued-lock operation as needed. If too many CPUs
>> are spinning on a given ticket lock, a queue structure will be allocated
>> and the lock will switch to queued-lock operation. When the lock becomes
>> free, it will switch back into ticketlock operation. The low-order bit
>> of the head counter is used to indicate that the lock is in queued mode,
>> which forces an unconditional mismatch between the head and tail counters.
>> This approach means that the common-case code path under conditions of
>> low contention is very nearly that of a plain ticket lock.
>>
>> A fixed number of queueing structures is statically allocated in an
>> array. The ticket-lock address is used to hash into an initial element,
>> but if that element is already in use, it moves to the next element. If
>> the entire array is already in use, continue to spin in ticket mode.
>>
>> This has been only lightly tested in the kernel, though a userspace
>> implementation has survived substantial testing.
>>
>> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>>
>> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
>> index 33692ea..b4a91b0 100644
>> --- a/arch/x86/include/asm/spinlock.h
>> +++ b/arch/x86/include/asm/spinlock.h
>> @@ -34,6 +34,8 @@
>> # define UNLOCK_LOCK_PREFIX
>> #endif
>>
>> +#ifndef CONFIG_TICKET_LOCK_QUEUED
>> +
>> /*
>> * Ticket locks are conceptually two parts, one indicating the current head of
>> * the queue, and the other indicating the current tail. The lock is acquired
>> @@ -62,6 +64,25 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>> barrier(); /* make sure nothing creeps before the lock is taken */
>> }
>>
>> +#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 */
>> +
>> static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
>> {
>> arch_spinlock_t old, new;
>> @@ -70,17 +91,37 @@ static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
>> if (old.tickets.head != old.tickets.tail)
>> return 0;
>>
>> +#ifndef CONFIG_TICKET_LOCK_QUEUED
>> new.head_tail = old.head_tail + (1 << TICKET_SHIFT);
>> +#else /* #ifndef CONFIG_TICKET_LOCK_QUEUED */
>> + new.head_tail = old.head_tail + (2 << TICKET_SHIFT);
>> +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
>>
>> /* cmpxchg is a full barrier, so nothing can move before it */
>> return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;
>> }
>>
>> +#ifndef CONFIG_TICKET_LOCK_QUEUED
>> +
>> static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
>> {
>> __add(&lock->tickets.head, 1, UNLOCK_LOCK_PREFIX);
>> }
>>
>> +#else /* #ifndef CONFIG_TICKET_LOCK_QUEUED */
>> +
>> +extern void tkt_q_do_wake(arch_spinlock_t *asp);
>> +
>> +static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
>> +{
>> + __ticket_t head = 2;
>> +
>> + head = xadd(&lock->tickets.head, 2);
>> + if (head & 0x1)
>> + tkt_q_do_wake(lock);
>> +}
>> +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
>> +
>> static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
>> {
>> struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
>> diff --git a/arch/x86/include/asm/spinlock_types.h b/arch/x86/include/asm/spinlock_types.h
>> index ad0ad07..cdaefdd 100644
>> --- a/arch/x86/include/asm/spinlock_types.h
>> +++ b/arch/x86/include/asm/spinlock_types.h
>> @@ -7,12 +7,18 @@
>>
>> #include <linux/types.h>
>>
>> -#if (CONFIG_NR_CPUS < 256)
>> +#if (CONFIG_NR_CPUS < 128)
>> typedef u8 __ticket_t;
>> typedef u16 __ticketpair_t;
>> -#else
>> +#define TICKET_T_CMP_GE(a, b) (UCHAR_MAX / 2 >= (unsigned char)((a) - (b)))
>> +#elif (CONFIG_NR_CPUS < 32768)
>> typedef u16 __ticket_t;
>> typedef u32 __ticketpair_t;
>> +#define TICKET_T_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
>> +#else
>> +typedef u32 __ticket_t;
>> +typedef u64 __ticketpair_t;
>> +#define TICKET_T_CMP_GE(a, b) (UINT_MAX / 2 >= (unsigned int)((a) - (b)))
>> #endif
>>
>> #define TICKET_SHIFT (sizeof(__ticket_t) * 8)
>> @@ -21,7 +27,11 @@ typedef struct arch_spinlock {
>> union {
>> __ticketpair_t head_tail;
>> struct __raw_tickets {
>> +#ifdef __BIG_ENDIAN__
>> + __ticket_t tail, head;
>> +#else /* #ifdef __BIG_ENDIAN__ */
>> __ticket_t head, tail;
>> +#endif /* #else #ifdef __BIG_ENDIAN__ */
>> } tickets;
>> };
>> } arch_spinlock_t;
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>> index e9ef6d6..816a87c 100644
>> --- a/include/linux/kernel.h
>> +++ b/include/linux/kernel.h
>> @@ -15,6 +15,7 @@
>> #include <asm/byteorder.h>
>> #include <uapi/linux/kernel.h>
>>
>> +#define UCHAR_MAX ((u8)(~0U))
>> #define USHRT_MAX ((u16)(~0U))
>> #define SHRT_MAX ((s16)(USHRT_MAX>>1))
>> #define SHRT_MIN ((s16)(-SHRT_MAX - 1))
>> diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
>> index 44511d1..ad9c67c 100644
>> --- a/kernel/Kconfig.locks
>> +++ b/kernel/Kconfig.locks
>> @@ -223,3 +223,21 @@ endif
>> config MUTEX_SPIN_ON_OWNER
>> def_bool y
>> depends on SMP && !DEBUG_MUTEXES
>> +
>> +config TICKET_LOCK_QUEUED
>> + bool "Dynamically switch between ticket and queued locking"
>> + default n
>> + ---help---
>> + Enable dynamic switching between ticketlock and queued locking
>> + on a per-lock basis. This option will slow down low-contention
>> + acquisition and release very slightly (additional conditional
>> + in release path), but will provide more efficient operation at
>> + high levels of lock contention. High-contention operation will
>> + not be quite as efficient as would be a pure queued lock, but
>> + this dynamic approach consumes less memory than queud locks
>> + and also runs faster at low levels of contention.
>> +
>> + Say "Y" if you are running on a large system with a workload
>> + that is likely to result in high levels of contention.
>> +
>> + Say "N" if you are unsure.
>> diff --git a/kernel/Makefile b/kernel/Makefile
>> index 271fd31..70a91f7 100644
>> --- a/kernel/Makefile
>> +++ b/kernel/Makefile
>> @@ -51,6 +51,7 @@ endif
>> obj-$(CONFIG_SMP) += spinlock.o
>> obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
>> obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
>> +obj-$(CONFIG_TICKET_LOCK_QUEUED) += tktqlock.o
>> obj-$(CONFIG_UID16) += uid16.o
>> obj-$(CONFIG_MODULES) += module.o
>> obj-$(CONFIG_MODULE_SIG) += module_signing.o modsign_pubkey.o modsign_certificate.o
>> diff --git a/kernel/tktqlock.c b/kernel/tktqlock.c
>> new file mode 100644
>> index 0000000..f01b760
>> --- /dev/null
>> +++ b/kernel/tktqlock.c
>> @@ -0,0 +1,333 @@
>> +/*
>> + * Queued ticket spinlocks.
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License as published by
>> + * the Free Software Foundation; either version 2 of the License, or
>> + * (at your option) any later version.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
>> + * GNU General Public License for more details.
>> + *
>> + * You should have received a copy of the GNU General Public License
>> + * along with this program; if not, write to the Free Software
>> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
>> + *
>> + * Copyright IBM Corporation, 2013
>> + *
>> + * Authors: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> + */
>> +#include <linux/types.h>
>> +#include <linux/kernel.h>
>> +#include <linux/spinlock.h>
>> +#include <linux/smp.h>
>> +#include <linux/percpu.h>
>> +
>> +struct tkt_q {
>> + int cpu;
>> + __ticket_t tail;
>> + struct tkt_q *next;
>> +};
>> +
>> +struct tkt_q_head {
>> + arch_spinlock_t *ref; /* Pointer to spinlock if in use. */
>> + s32 head_tkt; /* Head ticket when started queuing. */
>> + struct tkt_q *spin; /* Head of queue. */
>> + struct tkt_q **spin_tail; /* Tail of queue. */
>> +};
>> +
>> +/*
>> + * TKT_Q_SWITCH is twice the number of CPUs that must be spinning on a
>> + * given ticket lock to motivate switching to spinning on a queue.
>> + * The reason that it is twice the number is because the bottom bit of
>> + * the ticket is reserved for the bit that indicates that a queue is
>> + * associated with the lock.
>> + */
>> +#define TKT_Q_SWITCH (16 * 2)
>> +
>> +/*
>> + * TKT_Q_NQUEUES is the number of queues to maintain. Large systems
>> + * might have multiple highly contended locks, so provide more queues for
>> + * systems with larger numbers of CPUs.
>> + */
>> +#define TKT_Q_NQUEUES (DIV_ROUND_UP(NR_CPUS + TKT_Q_SWITCH - 1, TKT_Q_SWITCH) * 2)
>> +
>> +/* The queues themselves. */
>> +struct tkt_q_head tkt_q_heads[TKT_Q_NQUEUES];
>> +
>> +/* Advance to the next queue slot, wrapping around to the beginning. */
>> +static int tkt_q_next_slot(int i)
>> +{
>> + return (++i < TKT_Q_NQUEUES) ? i : 0;
>> +}
>> +
>> +/* Very crude hash from lock address to queue slot number. */
>> +static unsigned long tkt_q_hash(arch_spinlock_t *asp)
>> +{
>> + return (((unsigned long)asp) >> 8) % TKT_Q_NQUEUES;
>> +}
>> +
>> +/*
>> + * Return a pointer to the queue header associated with the specified lock,
>> + * or return NULL if there is no queue for the lock or if the lock's queue
>> + * is in transition.
>> + */
>> +static struct tkt_q_head *tkt_q_find_head(arch_spinlock_t *asp)
>> +{
>> + int i;
>> + int start;
>> +
>> + start = i = tkt_q_hash(asp);
>> + do
>> + if (tkt_q_heads[i].ref == asp)
>> + return &tkt_q_heads[i];
>> + while ((i = tkt_q_next_slot(i)) != start);
>> + return NULL;
>> +}
>> +
>> +/*
>> + * Try to stop queuing, reverting back to normal ticket-lock operation.
>> + * We can only stop queuing when the queue is empty, which means that
>> + * we need to correctly handle races where someone shows up in the queue
>> + * just as we are trying to dispense with the queue. They win, we lose.
>> + */
>> +static bool tkt_q_try_unqueue(arch_spinlock_t *asp, struct tkt_q_head *tqhp)
>> +{
>> + arch_spinlock_t asold;
>> + arch_spinlock_t asnew;
>> +
>> + /* Pick up the ticket values. */
>> + asold = ACCESS_ONCE(*asp);
>> + if ((asold.tickets.head & ~0x1) == asold.tickets.tail) {
>> +
>> + /* Attempt to mark the lock as not having a queue. */
>> + asnew = asold;
>> + asnew.tickets.head &= ~0x1;
>> + if (cmpxchg(&asp->head_tail,
>> + asold.head_tail,
>> + asnew.head_tail) == asold.head_tail) {
>> +
>> + /* Succeeded, mark the queue as unused. */
>> + ACCESS_ONCE(tqhp->ref) = NULL;
>> + return true;
>> + }
>> + }
>> +
>> + /* Failed, tell the caller there is still a queue to pass off to. */
>> + return false;
>> +}
>> +
>> +/*
>> + * Hand the lock off to the first CPU on the queue.
>> + */
>> +void tkt_q_do_wake(arch_spinlock_t *asp)
>> +{
>> + 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(asp)) == 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(asp, 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;
>> +}
>> +
>> +/*
>> + * 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 *asp, 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(asp);
>> + if (tqhp == NULL)
>> + return false;
>> +
>> + /* Initialize our queue element. */
>> + tq.cpu = raw_smp_processor_id();
>> + tq.tail = inc.tail;
>> + tq.next = NULL;
>> +
>> + /* 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. */
>> + 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 queue itself
>> + * at the head of 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. */
>> + 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 *asp, 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(asp->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(asp, inc);
>> + }
>> +
>> + /* The low-order bit in the head counter says "queued". */
>> + asnew.tickets.head |= 0x1;
>> + } while (cmpxchg(&asp->head_tail,
>> + asold.head_tail,
>> + asnew.head_tail) != asold.head_tail);
>> +
>> + /* Point the queue at the lock and go spin on it. */
>> + tqhp->head_tkt = asold.tickets.head;
>> + smp_mb(); /* Ensure head_tkt is set prior to queuers seeing tqhp. */
>> + ACCESS_ONCE(tqhp->ref) = asp;
>> + return tkt_q_do_spin(asp, inc);
>> +}
>
> Just small revise.
Sorry it is wrong. tkt_q_find_head() will returns wrong.
could we use only tkt_q_heads[tkt_q_hash(asp)] instead of find a free one?
>
> I just move " tqhp->head_tkt = asold.tickets.head;" into the loop, so
> we can use "asp->tickets.head & 0x1" to
> indicates that queued spinlock is prepared instead of by "tqhp->ref == asp".
>
> See the append diff.
> (And I guess, after it you can force only the CPUs which
> "inc.tail - tqhp->head_tkt > TKT_Q_SWITCH"
> do queued spin to remove the thundering herd)
>
> diff --git a/kernel/tktqlock.c b/kernel/tktqlock.c
> index f01b760..4ea409b 100644
> --- a/kernel/tktqlock.c
> +++ b/kernel/tktqlock.c
> @@ -27,7 +27,6 @@
>
> struct tkt_q {
> int cpu;
> - __ticket_t tail;
> struct tkt_q *next;
> };
>
> @@ -127,9 +126,8 @@ void tkt_q_do_wake(arch_spinlock_t *asp)
> 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(asp)) == NULL)
> - cpu_relax();
> + tqhp = tkt_q_find_head(asp);
> + BUG_ON(!tqhp);
>
> for (;;) {
>
> @@ -141,8 +139,6 @@ void tkt_q_do_wake(arch_spinlock_t *asp)
> 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;
> }
> @@ -164,20 +160,16 @@ bool tkt_q_do_spin(arch_spinlock_t *asp, struct
> __raw_tickets inc)
> */
> smp_mb(); /* See above block comment. */
>
> - /* If there no longer is a queue, leave. */
> tqhp = tkt_q_find_head(asp);
> - if (tqhp == NULL)
> - return false;
> + BUG_ON(!tqhp);
>
> /* Initialize our queue element. */
> tq.cpu = raw_smp_processor_id();
> - tq.tail = inc.tail;
> tq.next = NULL;
>
> /* 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;
> }
>
> @@ -251,16 +243,14 @@ tkt_q_init_contend(int i, arch_spinlock_t *asp,
> struct __raw_tickets inc)
> return tkt_q_do_spin(asp, inc);
> }
>
> + /* Point the queue at the lock and go spin on it. */
> + tqhp->head_tkt = asold.tickets.head;
> /* The low-order bit in the head counter says "queued". */
> asnew.tickets.head |= 0x1;
> } while (cmpxchg(&asp->head_tail,
> asold.head_tail,
> asnew.head_tail) != asold.head_tail);
>
> - /* Point the queue at the lock and go spin on it. */
> - tqhp->head_tkt = asold.tickets.head;
> - smp_mb(); /* Ensure head_tkt is set prior to queuers seeing tqhp. */
> - ACCESS_ONCE(tqhp->ref) = asp;
> return tkt_q_do_spin(asp, inc);
> }
>
> @@ -282,14 +272,9 @@ bool tkt_q_start_contend(arch_spinlock_t *asp,
> struct __raw_tickets inc)
> * 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 (cmpxchg(&tkt_q_heads[i].ref,
> NULL,
> - (arch_spinlock_t *)0x1) == NULL) {
> + asp) == NULL) {
>
> /* Succeeded, now go initialize it. */
> return tkt_q_init_contend(i, asp, inc);
if (tkt_q_heads[i].ref == asp)
--
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