summaryrefslogtreecommitdiffstats
path: root/include/asm-i386/rwsem.h
blob: 2f07601562e752a3a2646ffc67b8fba06e7c9c91 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
 *
 * Written by David Howells (dhowells@redhat.com).
 *
 * Derived from asm-i386/semaphore.h
 *
 *
 * The MSW of the count is the negated number of active writers and waiting
 * lockers, and the LSW is the total number of active locks
 *
 * The lock count is initialized to 0 (no active and no waiting lockers).
 *
 * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
 * uncontended lock. This can be determined because XADD returns the old value.
 * Readers increment by 1 and see a positive value when uncontended, negative
 * if there are writers (and maybe) readers waiting (in which case it goes to
 * sleep).
 *
 * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
 * be extended to 65534 by manually checking the whole MSW rather than relying
 * on the S flag.
 *
 * The value of ACTIVE_BIAS supports up to 65535 active processes.
 *
 * This should be totally fair - if anything is waiting, a process that wants a
 * lock will go to the back of the queue. When the currently active lock is
 * released, if there's a writer at the front of the queue, then that and only
 * that will be woken up; if there's a bunch of consequtive readers at the
 * front, then they'll all be woken up, but no other readers will be.
 */

#ifndef _I386_RWSEM_H
#define _I386_RWSEM_H

#ifndef _LINUX_RWSEM_H
#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
#endif

#ifdef __KERNEL__

#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/lockdep.h>

struct rwsem_waiter;

extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
extern struct rw_semaphore *FASTCALL(rwsem_downgrade_wake(struct rw_semaphore *sem));

/*
 * the semaphore definition
 */
struct rw_semaphore {
	signed long		count;
#define RWSEM_UNLOCKED_VALUE		0x00000000
#define RWSEM_ACTIVE_BIAS		0x00000001
#define RWSEM_ACTIVE_MASK		0x0000ffff
#define RWSEM_WAITING_BIAS		(-0x00010000)
#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS
#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
	spinlock_t		wait_lock;
	struct list_head	wait_list;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
	struct lockdep_map dep_map;
#endif
};

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
#else
# define __RWSEM_DEP_MAP_INIT(lockname)
#endif


#define __RWSEM_INITIALIZER(name) \
{ RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
	__RWSEM_DEP_MAP_INIT(name) }

#define DECLARE_RWSEM(name) \
	struct rw_semaphore name = __RWSEM_INITIALIZER(name)

extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
			 struct lock_class_key *key);

#define init_rwsem(sem)						\
do {								\
	static struct lock_class_key __key;			\
								\
	__init_rwsem((sem), #sem, &__key);			\
} while (0)

/*
 * lock for reading
 */
static inline void __down_read(struct rw_semaphore *sem)
{
	__asm__ __volatile__(
		"# beginning down_read\n\t"
LOCK_PREFIX	"  incl      (%%eax)\n\t" /* adds 0x00000001, returns the old value */
		"  js        2f\n\t" /* jump if we weren't granted the lock */
		"1:\n\t"
		LOCK_SECTION_START("")
		"2:\n\t"
		"  pushl     %%ecx\n\t"
		"  pushl     %%edx\n\t"
		"  call      rwsem_down_read_failed\n\t"
		"  popl      %%edx\n\t"
		"  popl      %%ecx\n\t"
		"  jmp       1b\n"
		LOCK_SECTION_END
		"# ending down_read\n\t"
		: "=m"(sem->count)
		: "a"(sem), "m"(sem->count)
		: "memory", "cc");
}

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
static inline int __down_read_trylock(struct rw_semaphore *sem)
{
	__s32 result, tmp;
	__asm__ __volatile__(
		"# beginning __down_read_trylock\n\t"
		"  movl      %0,%1\n\t"
		"1:\n\t"
		"  movl	     %1,%2\n\t"
		"  addl      %3,%2\n\t"
		"  jle	     2f\n\t"
LOCK_PREFIX	"  cmpxchgl  %2,%0\n\t"
		"  jnz	     1b\n\t"
		"2:\n\t"
		"# ending __down_read_trylock\n\t"
		: "+m"(sem->count), "=&a"(result), "=&r"(tmp)
		: "i"(RWSEM_ACTIVE_READ_BIAS)
		: "memory", "cc");
	return result>=0 ? 1 : 0;
}

/*
 * lock for writing
 */
static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
{
	int tmp;

	tmp = RWSEM_ACTIVE_WRITE_BIAS;
	__asm__ __volatile__(
		"# beginning down_write\n\t"
LOCK_PREFIX	"  xadd      %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
		"  testl     %%edx,%%edx\n\t" /* was the count 0 before? */
		"  jnz       2f\n\t" /* jump if we weren't granted the lock */
		"1:\n\t"
		LOCK_SECTION_START("")
		"2:\n\t"
		"  pushl     %%ecx\n\t"
		"  call      rwsem_down_write_failed\n\t"
		"  popl      %%ecx\n\t"
		"  jmp       1b\n"
		LOCK_SECTION_END
		"# ending down_write"
		: "=m"(sem->count), "=d"(tmp)
		: "a"(sem), "1"(tmp), "m"(sem->count)
		: "memory", "cc");
}

static inline void __down_write(struct rw_semaphore *sem)
{
	__down_write_nested(sem, 0);
}

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
static inline int __down_write_trylock(struct rw_semaphore *sem)
{
	signed long ret = cmpxchg(&sem->count,
				  RWSEM_UNLOCKED_VALUE, 
				  RWSEM_ACTIVE_WRITE_BIAS);
	if (ret == RWSEM_UNLOCKED_VALUE)
		return 1;
	return 0;
}

/*
 * unlock after reading
 */
static inline void __up_read(struct rw_semaphore *sem)
{
	__s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
	__asm__ __volatile__(
		"# beginning __up_read\n\t"
LOCK_PREFIX	"  xadd      %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
		"  js        2f\n\t" /* jump if the lock is being waited upon */
		"1:\n\t"
		LOCK_SECTION_START("")
		"2:\n\t"
		"  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
		"  jnz       1b\n\t"
		"  pushl     %%ecx\n\t"
		"  call      rwsem_wake\n\t"
		"  popl      %%ecx\n\t"
		"  jmp       1b\n"
		LOCK_SECTION_END
		"# ending __up_read\n"
		: "=m"(sem->count), "=d"(tmp)
		: "a"(sem), "1"(tmp), "m"(sem->count)
		: "memory", "cc");
}

/*
 * unlock after writing
 */
static inline void __up_write(struct rw_semaphore *sem)
{
	__asm__ __volatile__(
		"# beginning __up_write\n\t"
		"  movl      %2,%%edx\n\t"
LOCK_PREFIX	"  xaddl     %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
		"  jnz       2f\n\t" /* jump if the lock is being waited upon */
		"1:\n\t"
		LOCK_SECTION_START("")
		"2:\n\t"
		"  decw      %%dx\n\t" /* did the active count reduce to 0? */
		"  jnz       1b\n\t" /* jump back if not */
		"  pushl     %%ecx\n\t"
		"  call      rwsem_wake\n\t"
		"  popl      %%ecx\n\t"
		"  jmp       1b\n"
		LOCK_SECTION_END
		"# ending __up_write\n"
		: "=m"(sem->count)
		: "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS), "m"(sem->count)
		: "memory", "cc", "edx");
}

/*
 * downgrade write lock to read lock
 */
static inline void __downgrade_write(struct rw_semaphore *sem)
{
	__asm__ __volatile__(
		"# beginning __downgrade_write\n\t"
LOCK_PREFIX	"  addl      %2,(%%eax)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
		"  js        2f\n\t" /* jump if the lock is being waited upon */
		"1:\n\t"
		LOCK_SECTION_START("")
		"2:\n\t"
		"  pushl     %%ecx\n\t"
		"  pushl     %%edx\n\t"
		"  call      rwsem_downgrade_wake\n\t"
		"  popl      %%edx\n\t"
		"  popl      %%ecx\n\t"
		"  jmp       1b\n"
		LOCK_SECTION_END
		"# ending __downgrade_write\n"
		: "=m"(sem->count)
		: "a"(sem), "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
		: "memory", "cc");
}

/*
 * implement atomic add functionality
 */
static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
{
	__asm__ __volatile__(
LOCK_PREFIX	"addl %1,%0"
		: "=m"(sem->count)
		: "ir"(delta), "m"(sem->count));
}

/*
 * implement exchange and add functionality
 */
static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
{
	int tmp = delta;

	__asm__ __volatile__(
LOCK_PREFIX	"xadd %0,(%2)"
		: "+r"(tmp), "=m"(sem->count)
		: "r"(sem), "m"(sem->count)
		: "memory");

	return tmp+delta;
}

static inline int rwsem_is_locked(struct rw_semaphore *sem)
{
	return (sem->count != 0);
}

#endif /* __KERNEL__ */
#endif /* _I386_RWSEM_H */
OpenPOWER on IntegriCloud