1/* SPDX-License-Identifier: GPL-2.0-or-later */
  2/*
  3 * Queued spinlock
  4 *
  5 * A 'generic' spinlock implementation that is based on MCS locks. For an
  6 * architecture that's looking for a 'generic' spinlock, please first consider
  7 * ticket-lock.h and only come looking here when you've considered all the
  8 * constraints below and can show your hardware does actually perform better
  9 * with qspinlock.
 10 *
 11 * qspinlock relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no
 12 * weaker than RCtso if you're power), where regular code only expects atomic_t
 13 * to be RCpc.
 14 *
 15 * qspinlock relies on a far greater (compared to asm-generic/spinlock.h) set
 16 * of atomic operations to behave well together, please audit them carefully to
 17 * ensure they all have forward progress. Many atomic operations may default to
 18 * cmpxchg() loops which will not have good forward progress properties on
 19 * LL/SC architectures.
 20 *
 21 * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
 22 * do. Carefully read the patches that introduced
 23 * queued_fetch_set_pending_acquire().
 24 *
 25 * qspinlock also heavily relies on mixed size atomic operations, in specific
 26 * it requires architectures to have xchg16; something which many LL/SC
 27 * architectures need to implement as a 32bit and+or in order to satisfy the
 28 * forward progress guarantees mentioned above.
 29 *
 30 * Further reading on mixed size atomics that might be relevant:
 31 *
 32 *   http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
 33 *
 34 * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
 35 * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
 36 *
 37 * Authors: Waiman Long <waiman.long@hpe.com>
 38 */
 39#ifndef __ASM_GENERIC_QSPINLOCK_H
 40#define __ASM_GENERIC_QSPINLOCK_H
 41
 42#include <asm-generic/qspinlock_types.h>
 43#include <linux/atomic.h>
 44
 45#ifndef queued_spin_is_locked
 46/**
 47 * queued_spin_is_locked - is the spinlock locked?
 48 * @lock: Pointer to queued spinlock structure
 49 * Return: 1 if it is locked, 0 otherwise
 50 */
 51static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
 52{
 53	/*
 54	 * Any !0 state indicates it is locked, even if _Q_LOCKED_VAL
 55	 * isn't immediately observable.
 56	 */
 57	return atomic_read(&lock->val);
 58}
 59#endif
 60
 61/**
 62 * queued_spin_value_unlocked - is the spinlock structure unlocked?
 63 * @lock: queued spinlock structure
 64 * Return: 1 if it is unlocked, 0 otherwise
 65 *
 66 * N.B. Whenever there are tasks waiting for the lock, it is considered
 67 *      locked wrt the lockref code to avoid lock stealing by the lockref
 68 *      code and change things underneath the lock. This also allows some
 69 *      optimizations to be applied without conflict with lockref.
 70 */
 71static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
 72{
 73	return !lock.val.counter;
 74}
 75
 76/**
 77 * queued_spin_is_contended - check if the lock is contended
 78 * @lock : Pointer to queued spinlock structure
 79 * Return: 1 if lock contended, 0 otherwise
 80 */
 81static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
 82{
 83	return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
 84}
 85/**
 86 * queued_spin_trylock - try to acquire the queued spinlock
 87 * @lock : Pointer to queued spinlock structure
 88 * Return: 1 if lock acquired, 0 if failed
 89 */
 90static __always_inline int queued_spin_trylock(struct qspinlock *lock)
 91{
 92	int val = atomic_read(&lock->val);
 93
 94	if (unlikely(val))
 95		return 0;
 96
 97	return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
 98}
 99
100extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
101
102#ifndef queued_spin_lock
103/**
104 * queued_spin_lock - acquire a queued spinlock
105 * @lock: Pointer to queued spinlock structure
106 */
107static __always_inline void queued_spin_lock(struct qspinlock *lock)
108{
109	int val = 0;
110
111	if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
112		return;
113
114	queued_spin_lock_slowpath(lock, val);
115}
116#endif
117
118#ifndef queued_spin_unlock
119/**
120 * queued_spin_unlock - release a queued spinlock
121 * @lock : Pointer to queued spinlock structure
122 */
123static __always_inline void queued_spin_unlock(struct qspinlock *lock)
124{
125	/*
126	 * unlock() needs release semantics:
127	 */
128	smp_store_release(&lock->locked, 0);
129}
130#endif
131
132#ifndef virt_spin_lock
133static __always_inline bool virt_spin_lock(struct qspinlock *lock)
134{
135	return false;
136}
137#endif
138
139/*
140 * Remapping spinlock architecture specific functions to the corresponding
141 * queued spinlock functions.
142 */
143#define arch_spin_is_locked(l)		queued_spin_is_locked(l)
144#define arch_spin_is_contended(l)	queued_spin_is_contended(l)
145#define arch_spin_value_unlocked(l)	queued_spin_value_unlocked(l)
146#define arch_spin_lock(l)		queued_spin_lock(l)
147#define arch_spin_trylock(l)		queued_spin_trylock(l)
148#define arch_spin_unlock(l)		queued_spin_unlock(l)
149
150#endif /* __ASM_GENERIC_QSPINLOCK_H */