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1 | /* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for x86_64+ |
2 | * | |
3 | * Written by David Howells (dhowells@redhat.com). | |
4 | * Ported by Andi Kleen <ak@suse.de> to x86-64. | |
5 | * | |
6 | * Derived from asm-i386/semaphore.h and asm-i386/rwsem.h | |
7 | * | |
8 | * | |
9 | * The MSW of the count is the negated number of active writers and waiting | |
10 | * lockers, and the LSW is the total number of active locks | |
11 | * | |
12 | * The lock count is initialized to 0 (no active and no waiting lockers). | |
13 | * | |
14 | * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an | |
15 | * uncontended lock. This can be determined because XADD returns the old value. | |
16 | * Readers increment by 1 and see a positive value when uncontended, negative | |
17 | * if there are writers (and maybe) readers waiting (in which case it goes to | |
18 | * sleep). | |
19 | * | |
20 | * The value of WAITING_BIAS supports up to 32766 waiting processes. This can | |
21 | * be extended to 65534 by manually checking the whole MSW rather than relying | |
22 | * on the S flag. | |
23 | * | |
24 | * The value of ACTIVE_BIAS supports up to 65535 active processes. | |
25 | * | |
26 | * This should be totally fair - if anything is waiting, a process that wants a | |
27 | * lock will go to the back of the queue. When the currently active lock is | |
28 | * released, if there's a writer at the front of the queue, then that and only | |
29 | * that will be woken up; if there's a bunch of consecutive readers at the | |
30 | * front, then they'll all be woken up, but no other readers will be. | |
31 | */ | |
32 | ||
33 | #ifndef _X8664_RWSEM_H | |
34 | #define _X8664_RWSEM_H | |
35 | ||
36 | #ifndef _LINUX_RWSEM_H | |
37 | #error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead" | |
38 | #endif | |
39 | ||
40 | #ifdef __KERNEL__ | |
41 | ||
42 | #include <linux/list.h> | |
43 | #include <linux/spinlock.h> | |
44 | ||
45 | struct rwsem_waiter; | |
46 | ||
47 | extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem); | |
48 | extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem); | |
49 | extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *); | |
50 | extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem); | |
51 | ||
52 | /* | |
53 | * the semaphore definition | |
54 | */ | |
55 | struct rw_semaphore { | |
56 | signed int count; | |
57 | #define RWSEM_UNLOCKED_VALUE 0x00000000 | |
58 | #define RWSEM_ACTIVE_BIAS 0x00000001 | |
59 | #define RWSEM_ACTIVE_MASK 0x0000ffff | |
60 | #define RWSEM_WAITING_BIAS (-0x00010000) | |
61 | #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS | |
62 | #define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS) | |
63 | spinlock_t wait_lock; | |
64 | struct list_head wait_list; | |
65 | #if RWSEM_DEBUG | |
66 | int debug; | |
67 | #endif | |
68 | }; | |
69 | ||
70 | /* | |
71 | * initialisation | |
72 | */ | |
73 | #if RWSEM_DEBUG | |
74 | #define __RWSEM_DEBUG_INIT , 0 | |
75 | #else | |
76 | #define __RWSEM_DEBUG_INIT /* */ | |
77 | #endif | |
78 | ||
79 | #define __RWSEM_INITIALIZER(name) \ | |
80 | { RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \ | |
81 | __RWSEM_DEBUG_INIT } | |
82 | ||
83 | #define DECLARE_RWSEM(name) \ | |
84 | struct rw_semaphore name = __RWSEM_INITIALIZER(name) | |
85 | ||
86 | static inline void init_rwsem(struct rw_semaphore *sem) | |
87 | { | |
88 | sem->count = RWSEM_UNLOCKED_VALUE; | |
89 | spin_lock_init(&sem->wait_lock); | |
90 | INIT_LIST_HEAD(&sem->wait_list); | |
91 | #if RWSEM_DEBUG | |
92 | sem->debug = 0; | |
93 | #endif | |
94 | } | |
95 | ||
96 | /* | |
97 | * lock for reading | |
98 | */ | |
99 | static inline void __down_read(struct rw_semaphore *sem) | |
100 | { | |
101 | __asm__ __volatile__( | |
102 | "# beginning down_read\n\t" | |
103 | LOCK_PREFIX " incl (%%rdi)\n\t" /* adds 0x00000001, returns the old value */ | |
104 | " js 2f\n\t" /* jump if we weren't granted the lock */ | |
105 | "1:\n\t" | |
106 | LOCK_SECTION_START("") \ | |
107 | "2:\n\t" | |
108 | " call rwsem_down_read_failed_thunk\n\t" | |
109 | " jmp 1b\n" | |
110 | LOCK_SECTION_END \ | |
111 | "# ending down_read\n\t" | |
112 | : "+m"(sem->count) | |
113 | : "D"(sem) | |
114 | : "memory", "cc"); | |
115 | } | |
116 | ||
117 | ||
118 | /* | |
119 | * trylock for reading -- returns 1 if successful, 0 if contention | |
120 | */ | |
121 | static inline int __down_read_trylock(struct rw_semaphore *sem) | |
122 | { | |
123 | __s32 result, tmp; | |
124 | __asm__ __volatile__( | |
125 | "# 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; | |
139 | } | |
140 | ||
141 | ||
142 | /* | |
143 | * lock for writing | |
144 | */ | |
145 | static inline void __down_write(struct rw_semaphore *sem) | |
146 | { | |
147 | int tmp; | |
148 | ||
149 | tmp = RWSEM_ACTIVE_WRITE_BIAS; | |
150 | __asm__ __volatile__( | |
151 | "# beginning down_write\n\t" | |
152 | LOCK_PREFIX " xaddl %0,(%%rdi)\n\t" /* subtract 0x0000ffff, returns the old value */ | |
153 | " testl %0,%0\n\t" /* was the count 0 before? */ | |
154 | " jnz 2f\n\t" /* jump if we weren't granted the lock */ | |
155 | "1:\n\t" | |
156 | LOCK_SECTION_START("") | |
157 | "2:\n\t" | |
158 | " call rwsem_down_write_failed_thunk\n\t" | |
159 | " jmp 1b\n" | |
160 | LOCK_SECTION_END | |
161 | "# ending down_write" | |
162 | : "=&r" (tmp) | |
163 | : "0"(tmp), "D"(sem) | |
164 | : "memory", "cc"); | |
165 | } | |
166 | ||
167 | /* | |
168 | * trylock for writing -- returns 1 if successful, 0 if contention | |
169 | */ | |
170 | static inline int __down_write_trylock(struct rw_semaphore *sem) | |
171 | { | |
172 | signed long ret = cmpxchg(&sem->count, | |
173 | RWSEM_UNLOCKED_VALUE, | |
174 | RWSEM_ACTIVE_WRITE_BIAS); | |
175 | if (ret == RWSEM_UNLOCKED_VALUE) | |
176 | return 1; | |
177 | return 0; | |
178 | } | |
179 | ||
180 | /* | |
181 | * unlock after reading | |
182 | */ | |
183 | static inline void __up_read(struct rw_semaphore *sem) | |
184 | { | |
185 | __s32 tmp = -RWSEM_ACTIVE_READ_BIAS; | |
186 | __asm__ __volatile__( | |
187 | "# beginning __up_read\n\t" | |
188 | LOCK_PREFIX " xaddl %[tmp],(%%rdi)\n\t" /* subtracts 1, returns the old value */ | |
189 | " js 2f\n\t" /* jump if the lock is being waited upon */ | |
190 | "1:\n\t" | |
191 | LOCK_SECTION_START("") | |
192 | "2:\n\t" | |
193 | " decw %w[tmp]\n\t" /* do nothing if still outstanding active readers */ | |
194 | " jnz 1b\n\t" | |
195 | " call rwsem_wake_thunk\n\t" | |
196 | " jmp 1b\n" | |
197 | LOCK_SECTION_END | |
198 | "# ending __up_read\n" | |
199 | : "+m"(sem->count), [tmp] "+r" (tmp) | |
200 | : "D"(sem) | |
201 | : "memory", "cc"); | |
202 | } | |
203 | ||
204 | /* | |
205 | * unlock after writing | |
206 | */ | |
207 | static inline void __up_write(struct rw_semaphore *sem) | |
208 | { | |
209 | unsigned tmp; | |
210 | __asm__ __volatile__( | |
211 | "# beginning __up_write\n\t" | |
212 | " movl %[bias],%[tmp]\n\t" | |
213 | LOCK_PREFIX " xaddl %[tmp],(%%rdi)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */ | |
214 | " jnz 2f\n\t" /* jump if the lock is being waited upon */ | |
215 | "1:\n\t" | |
216 | LOCK_SECTION_START("") | |
217 | "2:\n\t" | |
218 | " decw %w[tmp]\n\t" /* did the active count reduce to 0? */ | |
219 | " jnz 1b\n\t" /* jump back if not */ | |
220 | " call rwsem_wake_thunk\n\t" | |
221 | " jmp 1b\n" | |
222 | LOCK_SECTION_END | |
223 | "# ending __up_write\n" | |
224 | : "+m"(sem->count), [tmp] "=r" (tmp) | |
225 | : "D"(sem), [bias] "i"(-RWSEM_ACTIVE_WRITE_BIAS) | |
226 | : "memory", "cc"); | |
227 | } | |
228 | ||
229 | /* | |
230 | * downgrade write lock to read lock | |
231 | */ | |
232 | static inline void __downgrade_write(struct rw_semaphore *sem) | |
233 | { | |
234 | __asm__ __volatile__( | |
235 | "# beginning __downgrade_write\n\t" | |
236 | LOCK_PREFIX " addl %[bias],(%%rdi)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */ | |
237 | " js 2f\n\t" /* jump if the lock is being waited upon */ | |
238 | "1:\n\t" | |
239 | LOCK_SECTION_START("") | |
240 | "2:\n\t" | |
241 | " call rwsem_downgrade_thunk\n" | |
242 | " jmp 1b\n" | |
243 | LOCK_SECTION_END | |
244 | "# ending __downgrade_write\n" | |
245 | : "=m"(sem->count) | |
246 | : "D"(sem), [bias] "i"(-RWSEM_WAITING_BIAS), "m"(sem->count) | |
247 | : "memory", "cc"); | |
248 | } | |
249 | ||
250 | /* | |
251 | * implement atomic add functionality | |
252 | */ | |
253 | static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem) | |
254 | { | |
255 | __asm__ __volatile__( | |
256 | LOCK_PREFIX "addl %1,%0" | |
257 | :"=m"(sem->count) | |
258 | :"ir"(delta), "m"(sem->count)); | |
259 | } | |
260 | ||
261 | /* | |
262 | * implement exchange and add functionality | |
263 | */ | |
264 | static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem) | |
265 | { | |
266 | int tmp = delta; | |
267 | ||
268 | __asm__ __volatile__( | |
269 | LOCK_PREFIX "xaddl %0,(%2)" | |
270 | : "=r"(tmp), "=m"(sem->count) | |
271 | : "r"(sem), "m"(sem->count), "0" (tmp) | |
272 | : "memory"); | |
273 | ||
274 | return tmp+delta; | |
275 | } | |
276 | ||
277 | #endif /* __KERNEL__ */ | |
278 | #endif /* _X8664_RWSEM_H */ |