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150 lines
4.7 KiB
150 lines
4.7 KiB
/* 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 <[email protected]> |
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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 !atomic_read(&lock.val); |
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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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