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 <asm/qrwlock.h>
24
25/*
26 * This internal data structure is used for optimizing access to some of
27 * the subfields within the atomic_t cnts.
28 */
29struct __qrwlock {
30	union {
31		atomic_t cnts;
32		struct {
33#ifdef __LITTLE_ENDIAN
34			u8 wmode;	/* Writer mode   */
35			u8 rcnts[3];	/* Reader counts */
36#else
37			u8 rcnts[3];	/* Reader counts */
38			u8 wmode;	/* Writer mode   */
39#endif
40		};
41	};
42	arch_spinlock_t	lock;
43};
44
45/**
46 * rspin_until_writer_unlock - inc reader count & spin until writer is gone
47 * @lock  : Pointer to queue rwlock structure
48 * @writer: Current queue rwlock writer status byte
49 *
50 * In interrupt context or at the head of the queue, the reader will just
51 * increment the reader count & wait until the writer releases the lock.
52 */
53static __always_inline void
54rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
55{
56	while ((cnts & _QW_WMASK) == _QW_LOCKED) {
57		cpu_relax_lowlatency();
58		cnts = atomic_read_acquire(&lock->cnts);
59	}
60}
61
62/**
63 * queued_read_lock_slowpath - acquire read lock of a queue rwlock
64 * @lock: Pointer to queue rwlock structure
65 * @cnts: Current qrwlock lock value
66 */
67void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
68{
69	/*
70	 * Readers come here when they cannot get the lock without waiting
71	 */
72	if (unlikely(in_interrupt())) {
73		/*
74		 * Readers in interrupt context will get the lock immediately
75		 * if the writer is just waiting (not holding the lock yet).
76		 * The rspin_until_writer_unlock() function returns immediately
77		 * in this case. Otherwise, they will spin (with ACQUIRE
78		 * semantics) until the lock is available without waiting in
79		 * the queue.
80		 */
81		rspin_until_writer_unlock(lock, cnts);
82		return;
83	}
84	atomic_sub(_QR_BIAS, &lock->cnts);
85
86	/*
87	 * Put the reader into the wait queue
88	 */
89	arch_spin_lock(&lock->wait_lock);
90
91	/*
92	 * The ACQUIRE semantics of the following spinning code ensure
93	 * that accesses can't leak upwards out of our subsequent critical
94	 * section in the case that the lock is currently held for write.
95	 */
96	cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
97	rspin_until_writer_unlock(lock, cnts);
98
99	/*
100	 * Signal the next one in queue to become queue head
101	 */
102	arch_spin_unlock(&lock->wait_lock);
103}
104EXPORT_SYMBOL(queued_read_lock_slowpath);
105
106/**
107 * queued_write_lock_slowpath - acquire write lock of a queue rwlock
108 * @lock : Pointer to queue rwlock structure
109 */
110void queued_write_lock_slowpath(struct qrwlock *lock)
111{
112	u32 cnts;
113
114	/* Put the writer into the wait queue */
115	arch_spin_lock(&lock->wait_lock);
116
117	/* Try to acquire the lock directly if no reader is present */
118	if (!atomic_read(&lock->cnts) &&
119	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
120		goto unlock;
121
122	/*
123	 * Set the waiting flag to notify readers that a writer is pending,
124	 * or wait for a previous writer to go away.
125	 */
126	for (;;) {
127		struct __qrwlock *l = (struct __qrwlock *)lock;
128
129		if (!READ_ONCE(l->wmode) &&
130		   (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
131			break;
132
133		cpu_relax_lowlatency();
134	}
135
136	/* When no more readers, set the locked flag */
137	for (;;) {
138		cnts = atomic_read(&lock->cnts);
139		if ((cnts == _QW_WAITING) &&
140		    (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
141					    _QW_LOCKED) == _QW_WAITING))
142			break;
143
144		cpu_relax_lowlatency();
145	}
146unlock:
147	arch_spin_unlock(&lock->wait_lock);
148}
149EXPORT_SYMBOL(queued_write_lock_slowpath);
150