1 /* SPDX-License-Identifier: GPL-2.0 */
2 /* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
3 *
4 * Written by David Howells (dhowells@redhat.com).
5 *
6 * Derived from asm-x86/semaphore.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 _ASM_X86_RWSEM_H
34 #define _ASM_X86_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 #include <asm/asm.h>
42
43 /*
44 * The bias values and the counter type limits the number of
45 * potential readers/writers to 32767 for 32 bits and 2147483647
46 * for 64 bits.
47 */
48
49 #ifdef CONFIG_X86_64
50 # define RWSEM_ACTIVE_MASK 0xffffffffL
51 #else
52 # define RWSEM_ACTIVE_MASK 0x0000ffffL
53 #endif
54
55 #define RWSEM_UNLOCKED_VALUE 0x00000000L
56 #define RWSEM_ACTIVE_BIAS 0x00000001L
57 #define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
58 #define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
59 #define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
60
61 /*
62 * lock for reading
63 */
64 #define ____down_read(sem, slow_path) \
65 ({ \
66 struct rw_semaphore* ret; \
67 asm volatile("# beginning down_read\n\t" \
68 LOCK_PREFIX _ASM_INC "(%[sem])\n\t" \
69 /* adds 0x00000001 */ \
70 " jns 1f\n" \
71 " call " slow_path "\n" \
72 "1:\n\t" \
73 "# ending down_read\n\t" \
74 : "+m" (sem->count), "=a" (ret), \
75 ASM_CALL_CONSTRAINT \
76 : [sem] "a" (sem) \
77 : "memory", "cc"); \
78 ret; \
79 })
80
__down_read(struct rw_semaphore * sem)81 static inline void __down_read(struct rw_semaphore *sem)
82 {
83 ____down_read(sem, "call_rwsem_down_read_failed");
84 }
85
__down_read_killable(struct rw_semaphore * sem)86 static inline int __down_read_killable(struct rw_semaphore *sem)
87 {
88 if (IS_ERR(____down_read(sem, "call_rwsem_down_read_failed_killable")))
89 return -EINTR;
90 return 0;
91 }
92
93 /*
94 * trylock for reading -- returns 1 if successful, 0 if contention
95 */
__down_read_trylock(struct rw_semaphore * sem)96 static inline bool __down_read_trylock(struct rw_semaphore *sem)
97 {
98 long result, tmp;
99 asm volatile("# beginning __down_read_trylock\n\t"
100 " mov %[count],%[result]\n\t"
101 "1:\n\t"
102 " mov %[result],%[tmp]\n\t"
103 " add %[inc],%[tmp]\n\t"
104 " jle 2f\n\t"
105 LOCK_PREFIX " cmpxchg %[tmp],%[count]\n\t"
106 " jnz 1b\n\t"
107 "2:\n\t"
108 "# ending __down_read_trylock\n\t"
109 : [count] "+m" (sem->count), [result] "=&a" (result),
110 [tmp] "=&r" (tmp)
111 : [inc] "i" (RWSEM_ACTIVE_READ_BIAS)
112 : "memory", "cc");
113 return result >= 0;
114 }
115
116 /*
117 * lock for writing
118 */
119 #define ____down_write(sem, slow_path) \
120 ({ \
121 long tmp; \
122 struct rw_semaphore* ret; \
123 \
124 asm volatile("# beginning down_write\n\t" \
125 LOCK_PREFIX " xadd %[tmp],(%[sem])\n\t" \
126 /* adds 0xffff0001, returns the old value */ \
127 " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
128 /* was the active mask 0 before? */\
129 " jz 1f\n" \
130 " call " slow_path "\n" \
131 "1:\n" \
132 "# ending down_write" \
133 : "+m" (sem->count), [tmp] "=d" (tmp), \
134 "=a" (ret), ASM_CALL_CONSTRAINT \
135 : [sem] "a" (sem), "[tmp]" (RWSEM_ACTIVE_WRITE_BIAS) \
136 : "memory", "cc"); \
137 ret; \
138 })
139
__down_write(struct rw_semaphore * sem)140 static inline void __down_write(struct rw_semaphore *sem)
141 {
142 ____down_write(sem, "call_rwsem_down_write_failed");
143 }
144
__down_write_killable(struct rw_semaphore * sem)145 static inline int __down_write_killable(struct rw_semaphore *sem)
146 {
147 if (IS_ERR(____down_write(sem, "call_rwsem_down_write_failed_killable")))
148 return -EINTR;
149
150 return 0;
151 }
152
153 /*
154 * trylock for writing -- returns 1 if successful, 0 if contention
155 */
__down_write_trylock(struct rw_semaphore * sem)156 static inline bool __down_write_trylock(struct rw_semaphore *sem)
157 {
158 bool result;
159 long tmp0, tmp1;
160 asm volatile("# beginning __down_write_trylock\n\t"
161 " mov %[count],%[tmp0]\n\t"
162 "1:\n\t"
163 " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
164 /* was the active mask 0 before? */
165 " jnz 2f\n\t"
166 " mov %[tmp0],%[tmp1]\n\t"
167 " add %[inc],%[tmp1]\n\t"
168 LOCK_PREFIX " cmpxchg %[tmp1],%[count]\n\t"
169 " jnz 1b\n\t"
170 "2:\n\t"
171 CC_SET(e)
172 "# ending __down_write_trylock\n\t"
173 : [count] "+m" (sem->count), [tmp0] "=&a" (tmp0),
174 [tmp1] "=&r" (tmp1), CC_OUT(e) (result)
175 : [inc] "er" (RWSEM_ACTIVE_WRITE_BIAS)
176 : "memory");
177 return result;
178 }
179
180 /*
181 * unlock after reading
182 */
__up_read(struct rw_semaphore * sem)183 static inline void __up_read(struct rw_semaphore *sem)
184 {
185 long tmp;
186 asm volatile("# beginning __up_read\n\t"
187 LOCK_PREFIX " xadd %[tmp],(%[sem])\n\t"
188 /* subtracts 1, returns the old value */
189 " jns 1f\n\t"
190 " call call_rwsem_wake\n" /* expects old value in %edx */
191 "1:\n"
192 "# ending __up_read\n"
193 : "+m" (sem->count), [tmp] "=d" (tmp)
194 : [sem] "a" (sem), "[tmp]" (-RWSEM_ACTIVE_READ_BIAS)
195 : "memory", "cc");
196 }
197
198 /*
199 * unlock after writing
200 */
__up_write(struct rw_semaphore * sem)201 static inline void __up_write(struct rw_semaphore *sem)
202 {
203 long tmp;
204 asm volatile("# beginning __up_write\n\t"
205 LOCK_PREFIX " xadd %[tmp],(%[sem])\n\t"
206 /* subtracts 0xffff0001, returns the old value */
207 " jns 1f\n\t"
208 " call call_rwsem_wake\n" /* expects old value in %edx */
209 "1:\n\t"
210 "# ending __up_write\n"
211 : "+m" (sem->count), [tmp] "=d" (tmp)
212 : [sem] "a" (sem), "[tmp]" (-RWSEM_ACTIVE_WRITE_BIAS)
213 : "memory", "cc");
214 }
215
216 /*
217 * downgrade write lock to read lock
218 */
__downgrade_write(struct rw_semaphore * sem)219 static inline void __downgrade_write(struct rw_semaphore *sem)
220 {
221 asm volatile("# beginning __downgrade_write\n\t"
222 LOCK_PREFIX _ASM_ADD "%[inc],(%[sem])\n\t"
223 /*
224 * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
225 * 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
226 */
227 " jns 1f\n\t"
228 " call call_rwsem_downgrade_wake\n"
229 "1:\n\t"
230 "# ending __downgrade_write\n"
231 : "+m" (sem->count)
232 : [sem] "a" (sem), [inc] "er" (-RWSEM_WAITING_BIAS)
233 : "memory", "cc");
234 }
235
236 #endif /* __KERNEL__ */
237 #endif /* _ASM_X86_RWSEM_H */
238
