linux/include/asm-generic/qspinlock.h

/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
 * Queued spinlock
 *
 * A 'generic' spinlock implementation that is based on MCS locks. For an
 * architecture that's looking for a 'generic' spinlock, please first consider
 * ticket-lock.h and only come looking here when you've considered all the
 * constraints below and can show your hardware does actually perform better
 * with qspinlock.
 *
 * qspinlock relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no
 * weaker than RCtso if you're power), where regular code only expects atomic_t
 * to be RCpc.
 *
 * qspinlock relies on a far greater (compared to asm-generic/spinlock.h) set
 * of atomic operations to behave well together, please audit them carefully to
 * ensure they all have forward progress. Many atomic operations may default to
 * cmpxchg() loops which will not have good forward progress properties on
 * LL/SC architectures.
 *
 * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
 * do. Carefully read the patches that introduced
 * queued_fetch_set_pending_acquire().
 *
 * qspinlock also heavily relies on mixed size atomic operations, in specific
 * it requires architectures to have xchg16; something which many LL/SC
 * architectures need to implement as a 32bit and+or in order to satisfy the
 * forward progress guarantees mentioned above.
 *
 * Further reading on mixed size atomics that might be relevant:
 *
 *   http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
 *
 * (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>
#include <linux/atomic.h>
#include <linux/tracepoint-defs.h>

#ifndef queued_spin_is_locked
/**
 * 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)
{
	/*
	 * Any !0 state indicates it is locked, even if _Q_LOCKED_VAL
	 * isn't immediately observable.
	 */
	return atomic_read(&lock->val);
}
#endif

/**
 * 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 !lock.val.counter;
}

/**
 * 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)
{
	int val = atomic_read(&lock->val);

	if (unlikely(val))
		return 0;

	return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
}

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

#ifndef queued_spin_lock
/**
 * queued_spin_lock - acquire a queued spinlock
 * @lock: Pointer to queued spinlock structure
 */
static __always_inline void queued_spin_lock(struct qspinlock *lock)
{
	int val = 0;

	if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
		return;

	queued_spin_lock_slowpath(lock, val);
}
#endif

#ifndef queued_spin_release
/**
 * queued_spin_release - release a queued spinlock
 * @lock : Pointer to queued spinlock structure
 */
static __always_inline void queued_spin_release(struct qspinlock *lock)
{
	/*
	 * unlock() needs release semantics:
	 */
	smp_store_release(&lock->locked, 0);
}
#endif

#ifndef queued_spin_unlock

DECLARE_TRACEPOINT(contended_release);

extern void queued_spin_release_traced(struct qspinlock *lock);

/**
 * queued_spin_unlock - unlock a queued spinlock
 * @lock : Pointer to queued spinlock structure
 *
 * Generic tracing wrapper around the arch-overridable
 * queued_spin_release().
 */
static __always_inline void queued_spin_unlock(struct qspinlock *lock)
{
	/*
	 * Trace and release are combined in queued_spin_release_traced() so
	 * the compiler does not need to preserve the lock pointer across the
	 * function call, avoiding callee-saved register save/restore on the
	 * hot path. queued_spin_release() is therefore called both here and in
	 * queued_spin_release_traced(). Keep the two in sync.
	 */
	if (IS_ENABLED(CONFIG_QUEUED_SPINLOCKS_TRACE_CONTENDED_RELEASE) &&
	    tracepoint_enabled(contended_release)) {
		queued_spin_release_traced(lock);
		return;
	}
	queued_spin_release(lock);
}
#endif

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

#ifndef __no_arch_spinlock_redefine
/*
 * 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)
#endif

#endif /* __ASM_GENERIC_QSPINLOCK_H */