151 lines · c
1/* SPDX-License-Identifier: GPL-2.0-or-later */2/*3 * Queued spinlock4 *5 * A 'generic' spinlock implementation that is based on MCS locks. For an6 * architecture that's looking for a 'generic' spinlock, please first consider7 * ticket-lock.h and only come looking here when you've considered all the8 * constraints below and can show your hardware does actually perform better9 * with qspinlock.10 *11 * qspinlock relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no12 * weaker than RCtso if you're power), where regular code only expects atomic_t13 * to be RCpc.14 *15 * qspinlock relies on a far greater (compared to asm-generic/spinlock.h) set16 * of atomic operations to behave well together, please audit them carefully to17 * ensure they all have forward progress. Many atomic operations may default to18 * cmpxchg() loops which will not have good forward progress properties on19 * LL/SC architectures.20 *21 * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)22 * do. Carefully read the patches that introduced23 * queued_fetch_set_pending_acquire().24 *25 * qspinlock also heavily relies on mixed size atomic operations, in specific26 * it requires architectures to have xchg16; something which many LL/SC27 * architectures need to implement as a 32bit and+or in order to satisfy the28 * 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.pdf33 *34 * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.35 * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP36 *37 * Authors: Waiman Long <waiman.long@hpe.com>38 */39#ifndef __ASM_GENERIC_QSPINLOCK_H40#define __ASM_GENERIC_QSPINLOCK_H41 42#include <asm-generic/qspinlock_types.h>43#include <linux/atomic.h>44 45#ifndef queued_spin_is_locked46/**47 * queued_spin_is_locked - is the spinlock locked?48 * @lock: Pointer to queued spinlock structure49 * Return: 1 if it is locked, 0 otherwise50 */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_VAL55 * isn't immediately observable.56 */57 return atomic_read(&lock->val);58}59#endif60 61/**62 * queued_spin_value_unlocked - is the spinlock structure unlocked?63 * @lock: queued spinlock structure64 * Return: 1 if it is unlocked, 0 otherwise65 *66 * N.B. Whenever there are tasks waiting for the lock, it is considered67 * locked wrt the lockref code to avoid lock stealing by the lockref68 * code and change things underneath the lock. This also allows some69 * 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 contended78 * @lock : Pointer to queued spinlock structure79 * Return: 1 if lock contended, 0 otherwise80 */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 spinlock87 * @lock : Pointer to queued spinlock structure88 * Return: 1 if lock acquired, 0 if failed89 */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_lock103/**104 * queued_spin_lock - acquire a queued spinlock105 * @lock: Pointer to queued spinlock structure106 */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#endif117 118#ifndef queued_spin_unlock119/**120 * queued_spin_unlock - release a queued spinlock121 * @lock : Pointer to queued spinlock structure122 */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#endif131 132#ifndef virt_spin_lock133static __always_inline bool virt_spin_lock(struct qspinlock *lock)134{135 return false;136}137#endif138 139/*140 * Remapping spinlock architecture specific functions to the corresponding141 * 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 */151