1 /* SPDX-License-Identifier: GPL-2.0-or-later */ 2 /* 3 * Resilient Queued Spin Lock defines 4 * 5 * (C) Copyright 2024-2025 Meta Platforms, Inc. and affiliates. 6 * 7 * Authors: Kumar Kartikeya Dwivedi <memxor@gmail.com> 8 */ 9 #ifndef __LINUX_RQSPINLOCK_H 10 #define __LINUX_RQSPINLOCK_H 11 12 #include "../locking/qspinlock.h" 13 14 /* 15 * try_cmpxchg_tail - Return result of cmpxchg of tail word with a new value 16 * @lock: Pointer to queued spinlock structure 17 * @tail: The tail to compare against 18 * @new_tail: The new queue tail code word 19 * Return: Bool to indicate whether the cmpxchg operation succeeded 20 * 21 * This is used by the head of the wait queue to clean up the queue. 22 * Provides relaxed ordering, since observers only rely on initialized 23 * state of the node which was made visible through the xchg_tail operation, 24 * i.e. through the smp_wmb preceding xchg_tail. 25 * 26 * We avoid using 16-bit cmpxchg, which is not available on all architectures. 27 */ 28 static __always_inline bool try_cmpxchg_tail(struct qspinlock *lock, u32 tail, u32 new_tail) 29 { 30 u32 old, new; 31 32 old = atomic_read(&lock->val); 33 do { 34 /* 35 * Is the tail part we compare to already stale? Fail. 36 */ 37 if ((old & _Q_TAIL_MASK) != tail) 38 return false; 39 /* 40 * Encode latest locked/pending state for new tail. 41 */ 42 new = (old & _Q_LOCKED_PENDING_MASK) | new_tail; 43 } while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new)); 44 45 return true; 46 } 47 48 #endif /* __LINUX_RQSPINLOCK_H */ 49