bit_spinlock.h

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#ifndef __LINUX_BIT_SPINLOCK_H
#define __LINUX_BIT_SPINLOCK_H

#include <linux/kernel.h>
#include <linux/preempt.h>
#include <linux/atomic.h>
#include <linux/bug.h>

/*
 *  bit-based spin_lock()
 *
 * Don't use this unless you really need to: spin_lock() and spin_unlock()
 * are significantly faster.
 */
static inline void bit_spin_lock(int bitnum, unsigned long *addr)
{
    /*
     * Assuming the lock is uncontended, this never enters
     * the body of the outer loop. If it is contended, then
     * within the inner loop a non-atomic test is used to
     * busywait with less bus contention for a good time to
     * attempt to acquire the lock bit.
     */
    preempt_disable();
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
    while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
        preempt_enable();
        do {
            cpu_relax();
        } while (test_bit(bitnum, addr));
        preempt_disable();
    }
#endif
    __acquire(bitlock);
}

/*
 * Return true if it was acquired
 */
static inline int bit_spin_trylock(int bitnum, unsigned long *addr)
{
    preempt_disable();
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
    if (unlikely(test_and_set_bit_lock(bitnum, addr))) {
        preempt_enable();
        return 0;
    }
#endif
    __acquire(bitlock);
    return 1;
}

/*
 *  bit-based spin_unlock()
 */
static inline void bit_spin_unlock(int bitnum, unsigned long *addr)
{
#ifdef CONFIG_DEBUG_SPINLOCK
    BUG_ON(!test_bit(bitnum, addr));
#endif
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
    clear_bit_unlock(bitnum, addr);
#endif
    preempt_enable();
    __release(bitlock);
}

/*
 *  bit-based spin_unlock()
 *  non-atomic version, which can be used eg. if the bit lock itself is
 *  protecting the rest of the flags in the word.
 */
static inline void __bit_spin_unlock(int bitnum, unsigned long *addr)
{
#ifdef CONFIG_DEBUG_SPINLOCK
    BUG_ON(!test_bit(bitnum, addr));
#endif
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
    __clear_bit_unlock(bitnum, addr);
#endif
    preempt_enable();
    __release(bitlock);
}

/*
 * Return true if the lock is held.
 */
static inline int bit_spin_is_locked(int bitnum, unsigned long *addr)
{
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
    return test_bit(bitnum, addr);
#elif defined CONFIG_PREEMPT_COUNT
    return preempt_count();
#else
    return 1;
#endif
}

#endif /* __LINUX_BIT_SPINLOCK_H */