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Linux/include/asm-x86_64/rwsem.h

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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                 : ""(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), "" (tmp)
272                 : "memory");
273 
274         return tmp+delta;
275 }
276 
277 #endif /* __KERNEL__ */
278 #endif /* _X8664_RWSEM_H */
279 

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