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 consequtive 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 int __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 ? 1 : 0;
97 }
98
99 /*
100 * lock for writing
101 */
__down_write_nested(struct rw_semaphore * sem,int subclass)102 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
103 {
104 long tmp;
105 asm volatile("# beginning down_write\n\t"
106 LOCK_PREFIX " xadd %1,(%2)\n\t"
107 /* adds 0xffff0001, returns the old value */
108 " test %1,%1\n\t"
109 /* was the count 0 before? */
110 " jz 1f\n"
111 " call call_rwsem_down_write_failed\n"
112 "1:\n"
113 "# ending down_write"
114 : "+m" (sem->count), "=d" (tmp)
115 : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)
116 : "memory", "cc");
117 }
118
__down_write(struct rw_semaphore * sem)119 static inline void __down_write(struct rw_semaphore *sem)
120 {
121 __down_write_nested(sem, 0);
122 }
123
124 /*
125 * trylock for writing -- returns 1 if successful, 0 if contention
126 */
__down_write_trylock(struct rw_semaphore * sem)127 static inline int __down_write_trylock(struct rw_semaphore *sem)
128 {
129 long ret = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
130 RWSEM_ACTIVE_WRITE_BIAS);
131 if (ret == RWSEM_UNLOCKED_VALUE)
132 return 1;
133 return 0;
134 }
135
136 /*
137 * unlock after reading
138 */
__up_read(struct rw_semaphore * sem)139 static inline void __up_read(struct rw_semaphore *sem)
140 {
141 long tmp;
142 asm volatile("# beginning __up_read\n\t"
143 LOCK_PREFIX " xadd %1,(%2)\n\t"
144 /* subtracts 1, returns the old value */
145 " jns 1f\n\t"
146 " call call_rwsem_wake\n" /* expects old value in %edx */
147 "1:\n"
148 "# ending __up_read\n"
149 : "+m" (sem->count), "=d" (tmp)
150 : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)
151 : "memory", "cc");
152 }
153
154 /*
155 * unlock after writing
156 */
__up_write(struct rw_semaphore * sem)157 static inline void __up_write(struct rw_semaphore *sem)
158 {
159 long tmp;
160 asm volatile("# beginning __up_write\n\t"
161 LOCK_PREFIX " xadd %1,(%2)\n\t"
162 /* subtracts 0xffff0001, returns the old value */
163 " jns 1f\n\t"
164 " call call_rwsem_wake\n" /* expects old value in %edx */
165 "1:\n\t"
166 "# ending __up_write\n"
167 : "+m" (sem->count), "=d" (tmp)
168 : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
169 : "memory", "cc");
170 }
171
172 /*
173 * downgrade write lock to read lock
174 */
__downgrade_write(struct rw_semaphore * sem)175 static inline void __downgrade_write(struct rw_semaphore *sem)
176 {
177 asm volatile("# beginning __downgrade_write\n\t"
178 LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
179 /*
180 * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
181 * 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
182 */
183 " jns 1f\n\t"
184 " call call_rwsem_downgrade_wake\n"
185 "1:\n\t"
186 "# ending __downgrade_write\n"
187 : "+m" (sem->count)
188 : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
189 : "memory", "cc");
190 }
191
192 /*
193 * implement atomic add functionality
194 */
rwsem_atomic_add(long delta,struct rw_semaphore * sem)195 static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)
196 {
197 asm volatile(LOCK_PREFIX _ASM_ADD "%1,%0"
198 : "+m" (sem->count)
199 : "er" (delta));
200 }
201
202 /*
203 * implement exchange and add functionality
204 */
rwsem_atomic_update(long delta,struct rw_semaphore * sem)205 static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
206 {
207 return delta + xadd(&sem->count, delta);
208 }
209
210 #endif /* __KERNEL__ */
211 #endif /* _ASM_X86_RWSEM_H */
212