rqspinlock.h

 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 */
28static __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 */