root/arch/x86/include/asm/spinlock.h

INCLUDED FROM

DEFINITIONS

1. __ticket_enter_slowpath
2. __ticket_lock_spinning
3. __ticket_unlock_kick
4. arch_spin_value_unlocked
5. arch_spin_lock
6. arch_spin_trylock
7. __ticket_unlock_slowpath
8. arch_spin_unlock
9. arch_spin_is_locked
10. arch_spin_is_contended
11. arch_spin_lock_flags
12. arch_spin_unlock_wait

#ifndef _ASM_X86_SPINLOCK_H
#define _ASM_X86_SPINLOCK_H

#include <linux/jump_label.h>
#include <linux/atomic.h>
#include <asm/page.h>
#include <asm/processor.h>
#include <linux/compiler.h>
#include <asm/paravirt.h>
#include <asm/bitops.h>

/*
 * Your basic SMP spinlocks, allowing only a single CPU anywhere
 *
 * Simple spin lock operations.  There are two variants, one clears IRQ's
 * on the local processor, one does not.
 *
 * These are fair FIFO ticket locks, which support up to 2^16 CPUs.
 *
 * (the type definitions are in asm/spinlock_types.h)
 */

#ifdef CONFIG_X86_32
# define LOCK_PTR_REG "a"
#else
# define LOCK_PTR_REG "D"
#endif

#if defined(CONFIG_X86_32) && (defined(CONFIG_X86_PPRO_FENCE))
/*
 * On PPro SMP, we use a locked operation to unlock
 * (PPro errata 66, 92)
 */
# define UNLOCK_LOCK_PREFIX LOCK_PREFIX
#else
# define UNLOCK_LOCK_PREFIX
#endif

/* How long a lock should spin before we consider blocking */
#define SPIN_THRESHOLD  (1 << 15)

extern struct static_key paravirt_ticketlocks_enabled;
static __always_inline bool static_key_false(struct static_key *key);

#ifdef CONFIG_PARAVIRT_SPINLOCKS

static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
{
        set_bit(0, (volatile unsigned long *)&lock->tickets.tail);
}

#else  /* !CONFIG_PARAVIRT_SPINLOCKS */
static __always_inline void __ticket_lock_spinning(arch_spinlock_t *lock,
                                                        __ticket_t ticket)
{
}
static inline void __ticket_unlock_kick(arch_spinlock_t *lock,
                                                        __ticket_t ticket)
{
}

#endif /* CONFIG_PARAVIRT_SPINLOCKS */

static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
{
        return lock.tickets.head == lock.tickets.tail;
}

/*
 * 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
 * by atomically noting the tail and incrementing it by one (thus adding
 * ourself to the queue and noting our position), then waiting until the head
 * becomes equal to the the initial value of the tail.
 *
 * We use an xadd covering *both* parts of the lock, to increment the tail and
 * also load the position of the head, which takes care of memory ordering
 * issues and should be optimal for the uncontended case. Note the tail must be
 * in the high part, because a wide xadd increment of the low part would carry
 * up and contaminate the high part.
 */
static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
{
        register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };

        inc = xadd(&lock->tickets, inc);
        if (likely(inc.head == inc.tail))
                goto out;

        inc.tail &= ~TICKET_SLOWPATH_FLAG;
        for (;;) {
                unsigned count = SPIN_THRESHOLD;

                do {
                        if (ACCESS_ONCE(lock->tickets.head) == inc.tail)
                                goto out;
                        cpu_relax();
                } while (--count);
                __ticket_lock_spinning(lock, inc.tail);
        }
out:    barrier();      /* make sure nothing creeps before the lock is taken */
}

static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
{
        arch_spinlock_t old, new;

        old.tickets = ACCESS_ONCE(lock->tickets);
        if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG))
                return 0;

        new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);

        /* 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;
}

static inline void __ticket_unlock_slowpath(arch_spinlock_t *lock,
                                            arch_spinlock_t old)
{
        arch_spinlock_t new;

        BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);

        /* Perform the unlock on the "before" copy */
        old.tickets.head += TICKET_LOCK_INC;

        /* Clear the slowpath flag */
        new.head_tail = old.head_tail & ~(TICKET_SLOWPATH_FLAG << TICKET_SHIFT);

        /*
         * If the lock is uncontended, clear the flag - use cmpxchg in
         * case it changes behind our back though.
         */
        if (new.tickets.head != new.tickets.tail ||
            cmpxchg(&lock->head_tail, old.head_tail,
                                        new.head_tail) != old.head_tail) {
                /*
                 * Lock still has someone queued for it, so wake up an
                 * appropriate waiter.
                 */
                __ticket_unlock_kick(lock, old.tickets.head);
        }
}

static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
{
        if (TICKET_SLOWPATH_FLAG &&
            static_key_false(&paravirt_ticketlocks_enabled)) {
                arch_spinlock_t prev;

                prev = *lock;
                add_smp(&lock->tickets.head, TICKET_LOCK_INC);

                /* add_smp() is a full mb() */

                if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
                        __ticket_unlock_slowpath(lock, prev);
        } else
                __add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
}

static inline int arch_spin_is_locked(arch_spinlock_t *lock)
{
        struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);

        return tmp.tail != tmp.head;
}

static inline int arch_spin_is_contended(arch_spinlock_t *lock)
{
        struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);

        return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
}
#define arch_spin_is_contended  arch_spin_is_contended

static __always_inline void arch_spin_lock_flags(arch_spinlock_t *lock,
                                                  unsigned long flags)
{
        arch_spin_lock(lock);
}

static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
{
        while (arch_spin_is_locked(lock))
                cpu_relax();
}

/*
 * Read-write spinlocks, allowing multiple readers
 * but only one writer.
 *
 * NOTE! it is quite common to have readers in interrupts
 * but no interrupt writers. For those circumstances we
 * can "mix" irq-safe locks - any writer needs to get a
 * irq-safe write-lock, but readers can get non-irqsafe
 * read-locks.
 *
 * On x86, we implement read-write locks using the generic qrwlock with
 * x86 specific optimization.
 */

#include <asm/qrwlock.h>

#define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
#define arch_write_lock_flags(lock, flags) arch_write_lock(lock)

#define arch_spin_relax(lock)   cpu_relax()
#define arch_read_relax(lock)   cpu_relax()
#define arch_write_relax(lock)  cpu_relax()

#endif /* _ASM_X86_SPINLOCK_H */