1 /* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
2 *
3 * Written by David Howells (dhowells@redhat.com).
4 *
5 * Derived from asm-x86/semaphore.h
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 consecutive readers at the
29 * front, then they'll all be woken up, but no other readers will be.
30 */
31
32 #ifndef _ASM_X86_RWSEM_H
33 #define _ASM_X86_RWSEM_H
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 #include <asm/asm.h>
41
42 /*
43 * The bias values and the counter type limits the number of
44 * potential readers/writers to 32767 for 32 bits and 2147483647
45 * for 64 bits.
46 */
47
48 #ifdef CONFIG_X86_64
49 # define RWSEM_ACTIVE_MASK 0xffffffffL
50 #else
51 # define RWSEM_ACTIVE_MASK 0x0000ffffL
52 #endif
53
54 #define RWSEM_UNLOCKED_VALUE 0x00000000L
55 #define RWSEM_ACTIVE_BIAS 0x00000001L
56 #define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
57 #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
58 #define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
59
60 /*
61 * lock for reading
62 */
__down_read(struct rw_semaphore * sem)63 static inline void __down_read(struct rw_semaphore *sem)
64 {
65 asm volatile("# beginning down_read\n\t"
66 LOCK_PREFIX _ASM_INC "(%1)\n\t"
67 /* adds 0x00000001 */
68 " jns 1f\n"
69 " call call_rwsem_down_read_failed\n"
70 "1:\n\t"
71 "# ending down_read\n\t"
72 : "+m" (sem->count)
73 : "a" (sem)
74 : "memory", "cc");
75 }
76
77 /*
78 * trylock for reading -- returns 1 if successful, 0 if contention
79 */
__down_read_trylock(struct rw_semaphore * sem)80 static inline bool __down_read_trylock(struct rw_semaphore *sem)
81 {
82 long result, tmp;
83 asm volatile("# beginning __down_read_trylock\n\t"
84 " mov %0,%1\n\t"
85 "1:\n\t"
86 " mov %1,%2\n\t"
87 " add %3,%2\n\t"
88 " jle 2f\n\t"
89 LOCK_PREFIX " cmpxchg %2,%0\n\t"
90 " jnz 1b\n\t"
91 "2:\n\t"
92 "# ending __down_read_trylock\n\t"
93 : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
94 : "i" (RWSEM_ACTIVE_READ_BIAS)
95 : "memory", "cc");
96 return result >= 0;
97 }
98
99 /*
100 * lock for writing
101 */
102 #define ____down_write(sem, slow_path) \
103 ({ \
104 long tmp; \
105 struct rw_semaphore* ret; \
106 \
107 asm volatile("# beginning down_write\n\t" \
108 LOCK_PREFIX " xadd %1,(%4)\n\t" \
109 /* adds 0xffff0001, returns the old value */ \
110 " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
111 /* was the active mask 0 before? */\
112 " jz 1f\n" \
113 " call " slow_path "\n" \
114 "1:\n" \
115 "# ending down_write" \
116 : "+m" (sem->count), "=d" (tmp), \
117 "=a" (ret), ASM_CALL_CONSTRAINT \
118 : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
119 : "memory", "cc"); \
120 ret; \
121 })
122
__down_write(struct rw_semaphore * sem)123 static inline void __down_write(struct rw_semaphore *sem)
124 {
125 ____down_write(sem, "call_rwsem_down_write_failed");
126 }
127
__down_write_killable(struct rw_semaphore * sem)128 static inline int __down_write_killable(struct rw_semaphore *sem)
129 {
130 if (IS_ERR(____down_write(sem, "call_rwsem_down_write_failed_killable")))
131 return -EINTR;
132
133 return 0;
134 }
135
136 /*
137 * trylock for writing -- returns 1 if successful, 0 if contention
138 */
__down_write_trylock(struct rw_semaphore * sem)139 static inline bool __down_write_trylock(struct rw_semaphore *sem)
140 {
141 bool result;
142 long tmp0, tmp1;
143 asm volatile("# beginning __down_write_trylock\n\t"
144 " mov %0,%1\n\t"
145 "1:\n\t"
146 " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
147 /* was the active mask 0 before? */
148 " jnz 2f\n\t"
149 " mov %1,%2\n\t"
150 " add %4,%2\n\t"
151 LOCK_PREFIX " cmpxchg %2,%0\n\t"
152 " jnz 1b\n\t"
153 "2:\n\t"
154 CC_SET(e)
155 "# ending __down_write_trylock\n\t"
156 : "+m" (sem->count), "=&a" (tmp0), "=&r" (tmp1),
157 CC_OUT(e) (result)
158 : "er" (RWSEM_ACTIVE_WRITE_BIAS)
159 : "memory");
160 return result;
161 }
162
163 /*
164 * unlock after reading
165 */
__up_read(struct rw_semaphore * sem)166 static inline void __up_read(struct rw_semaphore *sem)
167 {
168 long tmp;
169 asm volatile("# beginning __up_read\n\t"
170 LOCK_PREFIX " xadd %1,(%2)\n\t"
171 /* subtracts 1, returns the old value */
172 " jns 1f\n\t"
173 " call call_rwsem_wake\n" /* expects old value in %edx */
174 "1:\n"
175 "# ending __up_read\n"
176 : "+m" (sem->count), "=d" (tmp)
177 : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)
178 : "memory", "cc");
179 }
180
181 /*
182 * unlock after writing
183 */
__up_write(struct rw_semaphore * sem)184 static inline void __up_write(struct rw_semaphore *sem)
185 {
186 long tmp;
187 asm volatile("# beginning __up_write\n\t"
188 LOCK_PREFIX " xadd %1,(%2)\n\t"
189 /* subtracts 0xffff0001, returns the old value */
190 " jns 1f\n\t"
191 " call call_rwsem_wake\n" /* expects old value in %edx */
192 "1:\n\t"
193 "# ending __up_write\n"
194 : "+m" (sem->count), "=d" (tmp)
195 : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
196 : "memory", "cc");
197 }
198
199 /*
200 * downgrade write lock to read lock
201 */
__downgrade_write(struct rw_semaphore * sem)202 static inline void __downgrade_write(struct rw_semaphore *sem)
203 {
204 asm volatile("# beginning __downgrade_write\n\t"
205 LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
206 /*
207 * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
208 * 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
209 */
210 " jns 1f\n\t"
211 " call call_rwsem_downgrade_wake\n"
212 "1:\n\t"
213 "# ending __downgrade_write\n"
214 : "+m" (sem->count)
215 : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
216 : "memory", "cc");
217 }
218
219 #endif /* __KERNEL__ */
220 #endif /* _ASM_X86_RWSEM_H */
221