1/*
 2 * Queued read/write locks
 3 *
 4 * This program is free software; you can redistribute it and/or modify
 5 * it under the terms of the GNU General Public License as published by
 6 * the Free Software Foundation; either version 2 of the License, or
 7 * (at your option) any later version.
 8 *
 9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12 * GNU General Public License for more details.
13 *
14 * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
15 *
16 * Authors: Waiman Long <waiman.long@hp.com>
17 */
18#include <linux/smp.h>
19#include <linux/bug.h>
20#include <linux/cpumask.h>
21#include <linux/percpu.h>
22#include <linux/hardirq.h>
23#include <linux/spinlock.h>
24#include <asm/qrwlock.h>
25
26/**
27 * queued_read_lock_slowpath - acquire read lock of a queue rwlock
28 * @lock: Pointer to queue rwlock structure
29 */
30void queued_read_lock_slowpath(struct qrwlock *lock)
31{
32	/*
33	 * Readers come here when they cannot get the lock without waiting
34	 */
35	if (unlikely(in_interrupt())) {
36		/*
37		 * Readers in interrupt context will get the lock immediately
38		 * if the writer is just waiting (not holding the lock yet),
39		 * so spin with ACQUIRE semantics until the lock is available
40		 * without waiting in the queue.
41		 */
42		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
43		return;
44	}
45	atomic_sub(_QR_BIAS, &lock->cnts);
46
47	/*
48	 * Put the reader into the wait queue
49	 */
50	arch_spin_lock(&lock->wait_lock);
51	atomic_add(_QR_BIAS, &lock->cnts);
52
53	/*
54	 * The ACQUIRE semantics of the following spinning code ensure
55	 * that accesses can't leak upwards out of our subsequent critical
56	 * section in the case that the lock is currently held for write.
57	 */
58	atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
59
60	/*
61	 * Signal the next one in queue to become queue head
62	 */
63	arch_spin_unlock(&lock->wait_lock);
64}
65EXPORT_SYMBOL(queued_read_lock_slowpath);
66
67/**
68 * queued_write_lock_slowpath - acquire write lock of a queue rwlock
69 * @lock : Pointer to queue rwlock structure
70 */
71void queued_write_lock_slowpath(struct qrwlock *lock)
72{
73	/* Put the writer into the wait queue */
74	arch_spin_lock(&lock->wait_lock);
75
76	/* Try to acquire the lock directly if no reader is present */
77	if (!atomic_read(&lock->cnts) &&
78	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
79		goto unlock;
80
81	/* Set the waiting flag to notify readers that a writer is pending */
82	atomic_add(_QW_WAITING, &lock->cnts);
83
84	/* When no more readers or writers, set the locked flag */
85	do {
86		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
87	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
88					_QW_LOCKED) != _QW_WAITING);
89unlock:
90	arch_spin_unlock(&lock->wait_lock);
91}
92EXPORT_SYMBOL(queued_write_lock_slowpath);