1// SPDX-License-Identifier: GPL-2.0-or-later
 2/*
 3 * Queued read/write locks
 4 *
 5 * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
 6 *
 7 * Authors: Waiman Long <waiman.long@hp.com>
 8 */
 9#include <linux/smp.h>
10#include <linux/bug.h>
11#include <linux/cpumask.h>
12#include <linux/percpu.h>
13#include <linux/hardirq.h>
14#include <linux/spinlock.h>
15#include <asm/qrwlock.h>
16
17/**
18 * queued_read_lock_slowpath - acquire read lock of a queue rwlock
19 * @lock: Pointer to queue rwlock structure
20 */
21void queued_read_lock_slowpath(struct qrwlock *lock)
22{
23	/*
24	 * Readers come here when they cannot get the lock without waiting
25	 */
26	if (unlikely(in_interrupt())) {
27		/*
28		 * Readers in interrupt context will get the lock immediately
29		 * if the writer is just waiting (not holding the lock yet),
30		 * so spin with ACQUIRE semantics until the lock is available
31		 * without waiting in the queue.
32		 */
33		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
34		return;
35	}
36	atomic_sub(_QR_BIAS, &lock->cnts);
37
38	/*
39	 * Put the reader into the wait queue
40	 */
41	arch_spin_lock(&lock->wait_lock);
42	atomic_add(_QR_BIAS, &lock->cnts);
43
44	/*
45	 * The ACQUIRE semantics of the following spinning code ensure
46	 * that accesses can't leak upwards out of our subsequent critical
47	 * section in the case that the lock is currently held for write.
48	 */
49	atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
50
51	/*
52	 * Signal the next one in queue to become queue head
53	 */
54	arch_spin_unlock(&lock->wait_lock);
55}
56EXPORT_SYMBOL(queued_read_lock_slowpath);
57
58/**
59 * queued_write_lock_slowpath - acquire write lock of a queue rwlock
60 * @lock : Pointer to queue rwlock structure
61 */
62void queued_write_lock_slowpath(struct qrwlock *lock)
63{
64	/* Put the writer into the wait queue */
65	arch_spin_lock(&lock->wait_lock);
66
67	/* Try to acquire the lock directly if no reader is present */
68	if (!atomic_read(&lock->cnts) &&
69	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
70		goto unlock;
71
72	/* Set the waiting flag to notify readers that a writer is pending */
73	atomic_add(_QW_WAITING, &lock->cnts);
74
75	/* When no more readers or writers, set the locked flag */
76	do {
77		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
78	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
79					_QW_LOCKED) != _QW_WAITING);
80unlock:
81	arch_spin_unlock(&lock->wait_lock);
82}
83EXPORT_SYMBOL(queued_write_lock_slowpath);