151 lines
4.7 KiB
C
151 lines
4.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0-or-later */
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/*
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* Queued spinlock
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*
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* A 'generic' spinlock implementation that is based on MCS locks. For an
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* architecture that's looking for a 'generic' spinlock, please first consider
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* ticket-lock.h and only come looking here when you've considered all the
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* constraints below and can show your hardware does actually perform better
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* with qspinlock.
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*
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* qspinlock relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no
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* weaker than RCtso if you're power), where regular code only expects atomic_t
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* to be RCpc.
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*
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* qspinlock relies on a far greater (compared to asm-generic/spinlock.h) set
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* of atomic operations to behave well together, please audit them carefully to
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* ensure they all have forward progress. Many atomic operations may default to
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* cmpxchg() loops which will not have good forward progress properties on
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* LL/SC architectures.
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*
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* One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
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* do. Carefully read the patches that introduced
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* queued_fetch_set_pending_acquire().
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*
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* qspinlock also heavily relies on mixed size atomic operations, in specific
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* it requires architectures to have xchg16; something which many LL/SC
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* architectures need to implement as a 32bit and+or in order to satisfy the
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* forward progress guarantees mentioned above.
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*
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* Further reading on mixed size atomics that might be relevant:
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*
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* http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
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*
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* (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
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* (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
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*
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* Authors: Waiman Long <waiman.long@hpe.com>
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*/
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#ifndef __ASM_GENERIC_QSPINLOCK_H
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#define __ASM_GENERIC_QSPINLOCK_H
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#include <asm-generic/qspinlock_types.h>
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#include <linux/atomic.h>
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#ifndef queued_spin_is_locked
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/**
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* queued_spin_is_locked - is the spinlock locked?
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* @lock: Pointer to queued spinlock structure
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* Return: 1 if it is locked, 0 otherwise
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*/
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static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
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{
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/*
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* Any !0 state indicates it is locked, even if _Q_LOCKED_VAL
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* isn't immediately observable.
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*/
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return atomic_read(&lock->val);
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}
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#endif
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/**
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* queued_spin_value_unlocked - is the spinlock structure unlocked?
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* @lock: queued spinlock structure
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* Return: 1 if it is unlocked, 0 otherwise
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*
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* N.B. Whenever there are tasks waiting for the lock, it is considered
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* locked wrt the lockref code to avoid lock stealing by the lockref
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* code and change things underneath the lock. This also allows some
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* optimizations to be applied without conflict with lockref.
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*/
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static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
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{
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return !lock.val.counter;
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}
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/**
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* queued_spin_is_contended - check if the lock is contended
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* @lock : Pointer to queued spinlock structure
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* Return: 1 if lock contended, 0 otherwise
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*/
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static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
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{
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return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
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}
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/**
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* queued_spin_trylock - try to acquire the queued spinlock
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* @lock : Pointer to queued spinlock structure
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* Return: 1 if lock acquired, 0 if failed
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*/
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static __always_inline int queued_spin_trylock(struct qspinlock *lock)
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{
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int val = atomic_read(&lock->val);
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if (unlikely(val))
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return 0;
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return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
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}
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extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
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#ifndef queued_spin_lock
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/**
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* queued_spin_lock - acquire a queued spinlock
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* @lock: Pointer to queued spinlock structure
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*/
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static __always_inline void queued_spin_lock(struct qspinlock *lock)
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{
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int val = 0;
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if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
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return;
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queued_spin_lock_slowpath(lock, val);
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}
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#endif
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#ifndef queued_spin_unlock
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/**
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* queued_spin_unlock - release a queued spinlock
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* @lock : Pointer to queued spinlock structure
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*/
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static __always_inline void queued_spin_unlock(struct qspinlock *lock)
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{
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/*
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* unlock() needs release semantics:
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*/
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smp_store_release(&lock->locked, 0);
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}
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#endif
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#ifndef virt_spin_lock
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static __always_inline bool virt_spin_lock(struct qspinlock *lock)
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{
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return false;
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}
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#endif
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/*
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* Remapping spinlock architecture specific functions to the corresponding
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* queued spinlock functions.
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*/
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#define arch_spin_is_locked(l) queued_spin_is_locked(l)
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#define arch_spin_is_contended(l) queued_spin_is_contended(l)
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#define arch_spin_value_unlocked(l) queued_spin_value_unlocked(l)
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#define arch_spin_lock(l) queued_spin_lock(l)
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#define arch_spin_trylock(l) queued_spin_trylock(l)
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#define arch_spin_unlock(l) queued_spin_unlock(l)
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#endif /* __ASM_GENERIC_QSPINLOCK_H */
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