mutex.h revision 1.2 
1/*	$NetBSD: mutex.h,v 1.2 2007/03/09 05:40:08 thorpej Exp $	*/
2
3/*-
4 * Copyright (c) 2002, 2007 The NetBSD Foundation, Inc.
5 * All rights reserved.
6 *
7 * This code is derived from software contributed to The NetBSD Foundation
8 * by Jason R. Thorpe and Andrew Doran.
9 *
10 * Redistribution and use in source and binary forms, with or without
11 * modification, are permitted provided that the following conditions
12 * are met:
13 * 1. Redistributions of source code must retain the above copyright
14 *    notice, this list of conditions and the following disclaimer.
15 * 2. Redistributions in binary form must reproduce the above copyright
16 *    notice, this list of conditions and the following disclaimer in the
17 *    documentation and/or other materials provided with the distribution.
18 * 3. All advertising materials mentioning features or use of this software
19 *    must display the following acknowledgement:
20 *	This product includes software developed by the NetBSD
21 *	Foundation, Inc. and its contributors.
22 * 4. Neither the name of The NetBSD Foundation nor the names of its
23 *    contributors may be used to endorse or promote products derived
24 *    from this software without specific prior written permission.
25 *
26 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
27 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
28 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
29 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
30 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
31 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
32 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
33 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
34 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
35 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
36 * POSSIBILITY OF SUCH DAMAGE.
37 */
38
39#ifndef _ARM_MUTEX_H_
40#define	_ARM_MUTEX_H_
41
42/*
43 * The arm mutex implementation is troublesome, because arm lacks a
44 * compare-and-set operation.  However, there aren't MP arms.
45 * SMP for spin mutexes is easy - we don't need
46 * to know who owns the lock.  For adaptive mutexes, we need an aditional
47 * interlock.
48 *
49 * The locked byte set by the sparc 'ldstub' instruction is 0xff.  sparc
50 * kernels are always loaded above 0xe0000000, and the low 5 bits of any
51 * "struct lwp *" are always zero.  So, to record the lock owner, we only
52 * need 23 bits of space.  mtxa_owner contains the mutex owner's address
53 * shifted right by 5: the top three bits of which will always be 0xe,
54 * overlapping with the interlock at the top byte, which is always 0xff
55 * when the mutex is held.
56 *
57 * For a mutex acquisition, the owner field is set in two steps: first,
58 * acquire the interlock (top byte), and then test the owner's address.
59 * Once the owner field is non zero, it will appear that the mutex is held,
60 * by which LWP it does not matter: other LWPs competing for the lock will
61 * fall through to mutex_vector_enter(), and either spin or sleep.
62 *
63 * As a result there is no space for a waiters bit in the owner field.  No
64 * problem, because it would be hard to synchronise using one without a CAS
65 * operation.  Note that in order to do unlocked release of adaptive
66 * mutexes, we need the effect of MUTEX_SET_WAITERS() to be immediatley
67 * visible on the bus.  So, adaptive mutexes share the spin lock byte with
68 * spin mutexes (set with ldstub), but it is not treated as a lock in its
69 * own right, rather as a flag that can be atomically set or cleared.
70 *
71 * When releasing an adaptive mutex, we first clear the owners field, and
72 * then check to see if the waiters byte is set.  This ensures that there
73 * will always be someone to wake any sleeping waiters up (even it the mutex
74 * is acquired immediately after we release it, or if we are preempted
75 * immediatley after clearing the owners field).  The setting or clearing of
76 * the waiters byte is serialized by the turnstile chain lock associated
77 * with the mutex.
78 *
79 * See comments in kern_mutex.c about releasing adaptive mutexes without
80 * an interlocking step.
81 */
82
83#ifndef __MUTEX_PRIVATE
84
85struct kmutex {
86	uintptr_t	mtx_pad1;
87	uint32_t	mtx_pad2[4];
88};
89
90#else	/* __MUTEX_PRIVATE */
91
92struct kmutex {
93	volatile uintptr_t	mtx_owner;		/* 0-3 */
94	__cpu_simple_lock_t	mtx_interlock;		/* 4 */
95	__cpu_simple_lock_t	mtx_lock;		/* 5 */
96	uint8_t			mtx_pad[2];		/* 6-7 XXX */
97	ipl_cookie_t		mtx_ipl;		/* 8-11 */
98	uint32_t		mtx_id;			/* 16-19 */
99};
100
101#if 0
102#define	__HAVE_MUTEX_STUBS	1
103#define	__HAVE_SPIN_MUTEX_STUBS	1
104#endif
105
106static inline uintptr_t
107MUTEX_OWNER(uintptr_t owner)
108{
109	return owner;
110}
111
112static inline int
113MUTEX_OWNED(uintptr_t owner)
114{
115	return owner != 0;
116}
117
118static inline int
119MUTEX_SET_WAITERS(kmutex_t *mtx, uintptr_t owner)
120{
121	(void)__cpu_simple_lock_try(&mtx->mtx_lock);
122 	return mtx->mtx_owner != 0;
123}
124
125static inline void
126MUTEX_CLEAR_WAITERS(kmutex_t *mtx)
127{
128	__cpu_simple_unlock(&mtx->mtx_lock);
129}
130
131static inline int
132MUTEX_HAS_WAITERS(volatile kmutex_t *mtx)
133{
134	if (mtx->mtx_owner == 0)
135		return 0;
136	return mtx->mtx_lock == __SIMPLELOCK_LOCKED;
137}
138
139static inline void
140MUTEX_INITIALIZE_SPIN(kmutex_t *mtx, u_int id, int ipl)
141{
142	mtx->mtx_id = (id << 1) | 1;
143	mtx->mtx_ipl = makeiplcookie(ipl);
144	mtx->mtx_interlock = __SIMPLELOCK_LOCKED;
145	__cpu_simple_lock_init(&mtx->mtx_lock);
146}
147
148static inline void
149MUTEX_INITIALIZE_ADAPTIVE(kmutex_t *mtx, u_int id)
150{
151	mtx->mtx_id = (id << 1) | 0;
152	__cpu_simple_lock_init(&mtx->mtx_interlock);
153	__cpu_simple_lock_init(&mtx->mtx_lock);
154}
155
156static inline void
157MUTEX_DESTROY(kmutex_t *mtx)
158{
159	mtx->mtx_owner = (uintptr_t)-1L;
160	mtx->mtx_id = ~0;
161}
162
163static inline u_int
164MUTEX_GETID(kmutex_t *mtx)
165{
166	return mtx->mtx_id >> 1;
167}
168
169static inline bool
170MUTEX_SPIN_P(volatile kmutex_t *mtx)
171{
172	return (mtx->mtx_id & 1) == 1;
173}
174
175static inline bool
176MUTEX_ADAPTIVE_P(volatile kmutex_t *mtx)
177{
178	return (mtx->mtx_id & 1) == 0;
179}
180
181static inline int
182MUTEX_ACQUIRE(kmutex_t *mtx, uintptr_t curthread)
183{
184	if (!__cpu_simple_lock_try(&mtx->mtx_interlock))
185		return 0;
186	mtx->mtx_owner = curthread;
187	return 1;
188}
189
190static inline void
191MUTEX_RELEASE(kmutex_t *mtx)
192{
193	mtx->mtx_owner = 0;
194	__cpu_simple_unlock(&mtx->mtx_lock);
195	__cpu_simple_unlock(&mtx->mtx_interlock);
196}
197
198#endif	/* __MUTEX_PRIVATE */
199
200#endif /* _ARM_MUTEX_H_ */
201