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1da177e4 LT |
1 | /* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+ |
2 | * | |
3 | * Written by David Howells (dhowells@redhat.com). | |
4 | * | |
99122a3f | 5 | * Derived from asm-x86/semaphore.h |
1da177e4 LT |
6 | * |
7 | * | |
8 | * The MSW of the count is the negated number of active writers and waiting | |
9 | * lockers, and the LSW is the total number of active locks | |
10 | * | |
11 | * The lock count is initialized to 0 (no active and no waiting lockers). | |
12 | * | |
13 | * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an | |
14 | * uncontended lock. This can be determined because XADD returns the old value. | |
15 | * Readers increment by 1 and see a positive value when uncontended, negative | |
16 | * if there are writers (and maybe) readers waiting (in which case it goes to | |
17 | * sleep). | |
18 | * | |
19 | * The value of WAITING_BIAS supports up to 32766 waiting processes. This can | |
20 | * be extended to 65534 by manually checking the whole MSW rather than relying | |
21 | * on the S flag. | |
22 | * | |
23 | * The value of ACTIVE_BIAS supports up to 65535 active processes. | |
24 | * | |
25 | * This should be totally fair - if anything is waiting, a process that wants a | |
26 | * lock will go to the back of the queue. When the currently active lock is | |
27 | * released, if there's a writer at the front of the queue, then that and only | |
28 | * that will be woken up; if there's a bunch of consequtive readers at the | |
29 | * front, then they'll all be woken up, but no other readers will be. | |
30 | */ | |
31 | ||
77ef50a5 VN |
32 | #ifndef ASM_X86__RWSEM_H |
33 | #define ASM_X86__RWSEM_H | |
1da177e4 LT |
34 | |
35 | #ifndef _LINUX_RWSEM_H | |
36 | #error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead" | |
37 | #endif | |
38 | ||
39 | #ifdef __KERNEL__ | |
40 | ||
41 | #include <linux/list.h> | |
42 | #include <linux/spinlock.h> | |
4ea2176d | 43 | #include <linux/lockdep.h> |
1da177e4 LT |
44 | |
45 | struct rwsem_waiter; | |
46 | ||
d50efc6c IM |
47 | extern asmregparm struct rw_semaphore * |
48 | rwsem_down_read_failed(struct rw_semaphore *sem); | |
49 | extern asmregparm struct rw_semaphore * | |
50 | rwsem_down_write_failed(struct rw_semaphore *sem); | |
51 | extern asmregparm struct rw_semaphore * | |
52 | rwsem_wake(struct rw_semaphore *); | |
53 | extern asmregparm struct rw_semaphore * | |
54 | rwsem_downgrade_wake(struct rw_semaphore *sem); | |
1da177e4 LT |
55 | |
56 | /* | |
57 | * the semaphore definition | |
58 | */ | |
6e5609a9 | 59 | |
1da177e4 LT |
60 | #define RWSEM_UNLOCKED_VALUE 0x00000000 |
61 | #define RWSEM_ACTIVE_BIAS 0x00000001 | |
62 | #define RWSEM_ACTIVE_MASK 0x0000ffff | |
63 | #define RWSEM_WAITING_BIAS (-0x00010000) | |
64 | #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS | |
65 | #define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS) | |
6e5609a9 JP |
66 | |
67 | struct rw_semaphore { | |
68 | signed long count; | |
1da177e4 LT |
69 | spinlock_t wait_lock; |
70 | struct list_head wait_list; | |
4ea2176d IM |
71 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
72 | struct lockdep_map dep_map; | |
73 | #endif | |
1da177e4 LT |
74 | }; |
75 | ||
4ea2176d IM |
76 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
77 | # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname } | |
78 | #else | |
79 | # define __RWSEM_DEP_MAP_INIT(lockname) | |
80 | #endif | |
81 | ||
82 | ||
6e5609a9 JP |
83 | #define __RWSEM_INITIALIZER(name) \ |
84 | { \ | |
85 | RWSEM_UNLOCKED_VALUE, __SPIN_LOCK_UNLOCKED((name).wait_lock), \ | |
86 | LIST_HEAD_INIT((name).wait_list) __RWSEM_DEP_MAP_INIT(name) \ | |
87 | } | |
1da177e4 | 88 | |
6e5609a9 | 89 | #define DECLARE_RWSEM(name) \ |
1da177e4 LT |
90 | struct rw_semaphore name = __RWSEM_INITIALIZER(name) |
91 | ||
4ea2176d IM |
92 | extern void __init_rwsem(struct rw_semaphore *sem, const char *name, |
93 | struct lock_class_key *key); | |
94 | ||
95 | #define init_rwsem(sem) \ | |
96 | do { \ | |
97 | static struct lock_class_key __key; \ | |
98 | \ | |
99 | __init_rwsem((sem), #sem, &__key); \ | |
100 | } while (0) | |
1da177e4 LT |
101 | |
102 | /* | |
103 | * lock for reading | |
104 | */ | |
105 | static inline void __down_read(struct rw_semaphore *sem) | |
106 | { | |
6e5609a9 JP |
107 | asm volatile("# beginning down_read\n\t" |
108 | LOCK_PREFIX " incl (%%eax)\n\t" | |
109 | /* adds 0x00000001, returns the old value */ | |
110 | " jns 1f\n" | |
111 | " call call_rwsem_down_read_failed\n" | |
112 | "1:\n\t" | |
113 | "# ending down_read\n\t" | |
114 | : "+m" (sem->count) | |
115 | : "a" (sem) | |
116 | : "memory", "cc"); | |
1da177e4 LT |
117 | } |
118 | ||
119 | /* | |
120 | * trylock for reading -- returns 1 if successful, 0 if contention | |
121 | */ | |
122 | static inline int __down_read_trylock(struct rw_semaphore *sem) | |
123 | { | |
124 | __s32 result, tmp; | |
6e5609a9 JP |
125 | asm volatile("# beginning __down_read_trylock\n\t" |
126 | " movl %0,%1\n\t" | |
127 | "1:\n\t" | |
128 | " movl %1,%2\n\t" | |
129 | " addl %3,%2\n\t" | |
130 | " jle 2f\n\t" | |
131 | LOCK_PREFIX " cmpxchgl %2,%0\n\t" | |
132 | " jnz 1b\n\t" | |
133 | "2:\n\t" | |
134 | "# ending __down_read_trylock\n\t" | |
135 | : "+m" (sem->count), "=&a" (result), "=&r" (tmp) | |
136 | : "i" (RWSEM_ACTIVE_READ_BIAS) | |
137 | : "memory", "cc"); | |
138 | return result >= 0 ? 1 : 0; | |
1da177e4 LT |
139 | } |
140 | ||
141 | /* | |
142 | * lock for writing | |
143 | */ | |
4ea2176d | 144 | static inline void __down_write_nested(struct rw_semaphore *sem, int subclass) |
1da177e4 LT |
145 | { |
146 | int tmp; | |
147 | ||
148 | tmp = RWSEM_ACTIVE_WRITE_BIAS; | |
6e5609a9 JP |
149 | asm volatile("# beginning down_write\n\t" |
150 | LOCK_PREFIX " xadd %%edx,(%%eax)\n\t" | |
151 | /* subtract 0x0000ffff, returns the old value */ | |
152 | " testl %%edx,%%edx\n\t" | |
153 | /* was the count 0 before? */ | |
154 | " jz 1f\n" | |
155 | " call call_rwsem_down_write_failed\n" | |
156 | "1:\n" | |
157 | "# ending down_write" | |
158 | : "+m" (sem->count), "=d" (tmp) | |
159 | : "a" (sem), "1" (tmp) | |
160 | : "memory", "cc"); | |
1da177e4 LT |
161 | } |
162 | ||
4ea2176d IM |
163 | static inline void __down_write(struct rw_semaphore *sem) |
164 | { | |
165 | __down_write_nested(sem, 0); | |
166 | } | |
167 | ||
1da177e4 LT |
168 | /* |
169 | * trylock for writing -- returns 1 if successful, 0 if contention | |
170 | */ | |
171 | static inline int __down_write_trylock(struct rw_semaphore *sem) | |
172 | { | |
173 | signed long ret = cmpxchg(&sem->count, | |
6e5609a9 | 174 | RWSEM_UNLOCKED_VALUE, |
1da177e4 LT |
175 | RWSEM_ACTIVE_WRITE_BIAS); |
176 | if (ret == RWSEM_UNLOCKED_VALUE) | |
177 | return 1; | |
178 | return 0; | |
179 | } | |
180 | ||
181 | /* | |
182 | * unlock after reading | |
183 | */ | |
184 | static inline void __up_read(struct rw_semaphore *sem) | |
185 | { | |
186 | __s32 tmp = -RWSEM_ACTIVE_READ_BIAS; | |
6e5609a9 JP |
187 | asm volatile("# beginning __up_read\n\t" |
188 | LOCK_PREFIX " xadd %%edx,(%%eax)\n\t" | |
189 | /* subtracts 1, returns the old value */ | |
190 | " jns 1f\n\t" | |
191 | " call call_rwsem_wake\n" | |
192 | "1:\n" | |
193 | "# ending __up_read\n" | |
194 | : "+m" (sem->count), "=d" (tmp) | |
195 | : "a" (sem), "1" (tmp) | |
196 | : "memory", "cc"); | |
1da177e4 LT |
197 | } |
198 | ||
199 | /* | |
200 | * unlock after writing | |
201 | */ | |
202 | static inline void __up_write(struct rw_semaphore *sem) | |
203 | { | |
6e5609a9 JP |
204 | asm volatile("# beginning __up_write\n\t" |
205 | " movl %2,%%edx\n\t" | |
206 | LOCK_PREFIX " xaddl %%edx,(%%eax)\n\t" | |
207 | /* tries to transition | |
208 | 0xffff0001 -> 0x00000000 */ | |
209 | " jz 1f\n" | |
210 | " call call_rwsem_wake\n" | |
211 | "1:\n\t" | |
212 | "# ending __up_write\n" | |
213 | : "+m" (sem->count) | |
214 | : "a" (sem), "i" (-RWSEM_ACTIVE_WRITE_BIAS) | |
215 | : "memory", "cc", "edx"); | |
1da177e4 LT |
216 | } |
217 | ||
218 | /* | |
219 | * downgrade write lock to read lock | |
220 | */ | |
221 | static inline void __downgrade_write(struct rw_semaphore *sem) | |
222 | { | |
6e5609a9 JP |
223 | asm volatile("# beginning __downgrade_write\n\t" |
224 | LOCK_PREFIX " addl %2,(%%eax)\n\t" | |
225 | /* transitions 0xZZZZ0001 -> 0xYYYY0001 */ | |
226 | " jns 1f\n\t" | |
227 | " call call_rwsem_downgrade_wake\n" | |
228 | "1:\n\t" | |
229 | "# ending __downgrade_write\n" | |
230 | : "+m" (sem->count) | |
231 | : "a" (sem), "i" (-RWSEM_WAITING_BIAS) | |
232 | : "memory", "cc"); | |
1da177e4 LT |
233 | } |
234 | ||
235 | /* | |
236 | * implement atomic add functionality | |
237 | */ | |
238 | static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem) | |
239 | { | |
6e5609a9 JP |
240 | asm volatile(LOCK_PREFIX "addl %1,%0" |
241 | : "+m" (sem->count) | |
242 | : "ir" (delta)); | |
1da177e4 LT |
243 | } |
244 | ||
245 | /* | |
246 | * implement exchange and add functionality | |
247 | */ | |
248 | static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem) | |
249 | { | |
250 | int tmp = delta; | |
251 | ||
6e5609a9 JP |
252 | asm volatile(LOCK_PREFIX "xadd %0,%1" |
253 | : "+r" (tmp), "+m" (sem->count) | |
254 | : : "memory"); | |
1da177e4 | 255 | |
6e5609a9 | 256 | return tmp + delta; |
1da177e4 LT |
257 | } |
258 | ||
eb92f4ef RVR |
259 | static inline int rwsem_is_locked(struct rw_semaphore *sem) |
260 | { | |
261 | return (sem->count != 0); | |
262 | } | |
263 | ||
1da177e4 | 264 | #endif /* __KERNEL__ */ |
77ef50a5 | 265 | #endif /* ASM_X86__RWSEM_H */ |