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    SubjectRe: [PATCH RFC ticketlock] Auto-queued ticketlock
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    On Tue, Jun 11, 2013 at 10:48 PM, Lai Jiangshan <eag0628@gmail.com> wrote:
    > On Mon, Jun 10, 2013 at 3:36 AM, Paul E. McKenney
    > <paulmck@linux.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@linux.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@linux.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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