linux/include/asm-generic/qspinlock.h

/*
 * Queued spinlock
 *
 * 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.
 *
 * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
 * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
 *
 * Authors: Waiman Long <waiman.long@hpe.com>
 */
#ifndef __ASM_GENERIC_QSPINLOCK_H
#define __ASM_GENERIC_QSPINLOCK_H

#include <asm-generic/qspinlock_types.h>

/**
 * queued_spin_is_locked - is the spinlock locked?
 * @lock: Pointer to queued spinlock structure
 * Return: 1 if it is locked, 0 otherwise
 */
static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
{
	/*
	 * queued_spin_lock_slowpath() can ACQUIRE the lock before
	 * issuing the unordered store that sets _Q_LOCKED_VAL.
	 *
	 * See both smp_cond_acquire() sites for more detail.
	 *
	 * This however means that in code like:
	 *
	 *   spin_lock(A)		spin_lock(B)
	 *   spin_unlock_wait(B)	spin_is_locked(A)
	 *   do_something()		do_something()
	 *
	 * Both CPUs can end up running do_something() because the store
	 * setting _Q_LOCKED_VAL will pass through the loads in
	 * spin_unlock_wait() and/or spin_is_locked().
	 *
	 * Avoid this by issuing a full memory barrier between the spin_lock()
	 * and the loads in spin_unlock_wait() and spin_is_locked().
	 *
	 * Note that regular mutual exclusion doesn't care about this
	 * delayed store.
	 */
	smp_mb();
	return atomic_read(&lock->val) & _Q_LOCKED_MASK;
}

/**
 * queued_spin_value_unlocked - is the spinlock structure unlocked?
 * @lock: queued spinlock structure
 * Return: 1 if it is unlocked, 0 otherwise
 *
 * N.B. Whenever there are tasks waiting for the lock, it is considered
 *      locked wrt the lockref code to avoid lock stealing by the lockref
 *      code and change things underneath the lock. This also allows some
 *      optimizations to be applied without conflict with lockref.
 */
static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
{
	return !atomic_read(&lock.val);
}

/**
 * queued_spin_is_contended - check if the lock is contended
 * @lock : Pointer to queued spinlock structure
 * Return: 1 if lock contended, 0 otherwise
 */
static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
{
	return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
}
/**
 * queued_spin_trylock - try to acquire the queued spinlock
 * @lock : Pointer to queued spinlock structure
 * Return: 1 if lock acquired, 0 if failed
 */
static __always_inline int queued_spin_trylock(struct qspinlock *lock)
{
	if (!atomic_read(&lock->val) &&
	   (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
		return 1;
	return 0;
}

extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);

/**
 * queued_spin_lock - acquire a queued spinlock
 * @lock: Pointer to queued spinlock structure
 */
static __always_inline void queued_spin_lock(struct qspinlock *lock)
{
	u32 val;

	val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
	if (likely(val == 0))
		return;
	queued_spin_lock_slowpath(lock, val);
}

#ifndef queued_spin_unlock
/**
 * queued_spin_unlock - release a queued spinlock
 * @lock : Pointer to queued spinlock structure
 */
static __always_inline void queued_spin_unlock(struct qspinlock *lock)
{
	/*
	 * smp_mb__before_atomic() in order to guarantee release semantics
	 */
	smp_mb__before_atomic();
	atomic_sub(_Q_LOCKED_VAL, &lock->val);
}
#endif

/**
 * queued_spin_unlock_wait - wait until current lock holder releases the lock
 * @lock : Pointer to queued spinlock structure
 *
 * There is a very slight possibility of live-lock if the lockers keep coming
 * and the waiter is just unfortunate enough to not see any unlock state.
 */
static inline void queued_spin_unlock_wait(struct qspinlock *lock)
{
	/* See queued_spin_is_locked() */
	smp_mb();
	while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
		cpu_relax();
}

#ifndef virt_spin_lock
static __always_inline bool virt_spin_lock(struct qspinlock *lock)
{
	return false;
}
#endif

/*
 * Remapping spinlock architecture specific functions to the corresponding
 * queued spinlock functions.
 */
#define arch_spin_is_locked(l)		queued_spin_is_locked(l)
#define arch_spin_is_contended(l)	queued_spin_is_contended(l)
#define arch_spin_value_unlocked(l)	queued_spin_value_unlocked(l)
#define arch_spin_lock(l)		queued_spin_lock(l)
#define arch_spin_trylock(l)		queued_spin_trylock(l)
#define arch_spin_unlock(l)		queued_spin_unlock(l)
#define arch_spin_lock_flags(l, f)	queued_spin_lock(l)
#define arch_spin_unlock_wait(l)	queued_spin_unlock_wait(l)

#endif /* __ASM_GENERIC_QSPINLOCK_H */