include/asm-i386/rwsem.h

   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-i386/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 _I386_RWSEM_H
  33 #define _I386_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
  41 #include <linux/list.h>
  42 #include <linux/spinlock.h>
  43
  44 struct rwsem_waiter;
  45
  46 extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
  47 extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
  48 extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
  49 extern struct rw_semaphore *FASTCALL(rwsem_downgrade_wake(struct rw_semaphore *sem));
  50
  51 /*
  52  * the semaphore definition
  53  */
  54 struct rw_semaphore {
  55         signed long             count;
  56 #define RWSEM_UNLOCKED_VALUE            0x00000000
  57 #define RWSEM_ACTIVE_BIAS               0x00000001
  58 #define RWSEM_ACTIVE_MASK               0x0000ffff
  59 #define RWSEM_WAITING_BIAS              (-0x00010000)
  60 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  61 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  62         spinlock_t              wait_lock;
  63         struct list_head        wait_list;
  64 };
  65
  66 #define __RWSEM_INITIALIZER(name) \
  67 { RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
  68         }
  69
  70 #define DECLARE_RWSEM(name) \
  71         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  72
  73 static inline void init_rwsem(struct rw_semaphore *sem)
  74 {
  75         sem->count = RWSEM_UNLOCKED_VALUE;
  76         spin_lock_init(&sem->wait_lock);
  77         INIT_LIST_HEAD(&sem->wait_list);
  78 }
  79
  80 /*
  81  * lock for reading
  82  */
  83 static inline void __down_read(struct rw_semaphore *sem)
  84 {
  85         __asm__ __volatile__(
  86                 "# beginning down_read\n\t"
  87 LOCK_PREFIX     "  incl      (%%eax)\n\t" /* adds 0x00000001, returns the old value */
  88                 "  js        2f\n\t" /* jump if we weren't granted the lock */
  89                 "1:\n\t"
  90                 LOCK_SECTION_START("")
  91                 "2:\n\t"
  92                 "  pushl     %%ecx\n\t"
  93                 "  pushl     %%edx\n\t"
  94                 "  call      rwsem_down_read_failed\n\t"
  95                 "  popl      %%edx\n\t"
  96                 "  popl      %%ecx\n\t"
  97                 "  jmp       1b\n"
  98                 LOCK_SECTION_END
  99                 "# ending down_read\n\t"
 100                 : "=m"(sem->count)
 101                 : "a"(sem), "m"(sem->count)
 102                 : "memory", "cc");
 103 }
 104
 105 /*
 106  * trylock for reading -- returns 1 if successful, 0 if contention
 107  */
 108 static inline int __down_read_trylock(struct rw_semaphore *sem)
 109 {
 110         __s32 result, tmp;
 111         __asm__ __volatile__(
 112                 "# beginning __down_read_trylock\n\t"
 113                 "  movl      %0,%1\n\t"
 114                 "1:\n\t"
 115                 "  movl      %1,%2\n\t"
 116                 "  addl      %3,%2\n\t"
 117                 "  jle       2f\n\t"
 118 LOCK_PREFIX     "  cmpxchgl  %2,%0\n\t"
 119                 "  jnz       1b\n\t"
 120                 "2:\n\t"
 121                 "# ending __down_read_trylock\n\t"
 122                 : "+m"(sem->count), "=&a"(result), "=&r"(tmp)
 123                 : "i"(RWSEM_ACTIVE_READ_BIAS)
 124                 : "memory", "cc");
 125         return result>=0 ? 1 : 0;
 126 }
 127
 128 /*
 129  * lock for writing
 130  */
 131 static inline void __down_write(struct rw_semaphore *sem)
 132 {
 133         int tmp;
 134
 135         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 136         __asm__ __volatile__(
 137                 "# beginning down_write\n\t"
 138 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
 139                 "  testl     %%edx,%%edx\n\t" /* was the count 0 before? */
 140                 "  jnz       2f\n\t" /* jump if we weren't granted the lock */
 141                 "1:\n\t"
 142                 LOCK_SECTION_START("")
 143                 "2:\n\t"
 144                 "  pushl     %%ecx\n\t"
 145                 "  call      rwsem_down_write_failed\n\t"
 146                 "  popl      %%ecx\n\t"
 147                 "  jmp       1b\n"
 148                 LOCK_SECTION_END
 149                 "# ending down_write"
 150                 : "=m"(sem->count), "=d"(tmp)
 151                 : "a"(sem), "1"(tmp), "m"(sem->count)
 152                 : "memory", "cc");
 153 }
 154
 155 /*
 156  * trylock for writing -- returns 1 if successful, 0 if contention
 157  */
 158 static inline int __down_write_trylock(struct rw_semaphore *sem)
 159 {
 160         signed long ret = cmpxchg(&sem->count,
 161                                   RWSEM_UNLOCKED_VALUE,
 162                                   RWSEM_ACTIVE_WRITE_BIAS);
 163         if (ret == RWSEM_UNLOCKED_VALUE)
 164                 return 1;
 165         return 0;
 166 }
 167
 168 /*
 169  * unlock after reading
 170  */
 171 static inline void __up_read(struct rw_semaphore *sem)
 172 {
 173         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 174         __asm__ __volatile__(
 175                 "# beginning __up_read\n\t"
 176 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
 177                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 178                 "1:\n\t"
 179                 LOCK_SECTION_START("")
 180                 "2:\n\t"
 181                 "  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
 182                 "  jnz       1b\n\t"
 183                 "  pushl     %%ecx\n\t"
 184                 "  call      rwsem_wake\n\t"
 185                 "  popl      %%ecx\n\t"
 186                 "  jmp       1b\n"
 187                 LOCK_SECTION_END
 188                 "# ending __up_read\n"
 189                 : "=m"(sem->count), "=d"(tmp)
 190                 : "a"(sem), "1"(tmp), "m"(sem->count)
 191                 : "memory", "cc");
 192 }
 193
 194 /*
 195  * unlock after writing
 196  */
 197 static inline void __up_write(struct rw_semaphore *sem)
 198 {
 199         __asm__ __volatile__(
 200                 "# beginning __up_write\n\t"
 201                 "  movl      %2,%%edx\n\t"
 202 LOCK_PREFIX     "  xaddl     %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
 203                 "  jnz       2f\n\t" /* jump if the lock is being waited upon */
 204                 "1:\n\t"
 205                 LOCK_SECTION_START("")
 206                 "2:\n\t"
 207                 "  decw      %%dx\n\t" /* did the active count reduce to 0? */
 208                 "  jnz       1b\n\t" /* jump back if not */
 209                 "  pushl     %%ecx\n\t"
 210                 "  call      rwsem_wake\n\t"
 211                 "  popl      %%ecx\n\t"
 212                 "  jmp       1b\n"
 213                 LOCK_SECTION_END
 214                 "# ending __up_write\n"
 215                 : "=m"(sem->count)
 216                 : "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS), "m"(sem->count)
 217                 : "memory", "cc", "edx");
 218 }
 219
 220 /*
 221  * downgrade write lock to read lock
 222  */
 223 static inline void __downgrade_write(struct rw_semaphore *sem)
 224 {
 225         __asm__ __volatile__(
 226                 "# beginning __downgrade_write\n\t"
 227 LOCK_PREFIX     "  addl      %2,(%%eax)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
 228                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 229                 "1:\n\t"
 230                 LOCK_SECTION_START("")
 231                 "2:\n\t"
 232                 "  pushl     %%ecx\n\t"
 233                 "  pushl     %%edx\n\t"
 234                 "  call      rwsem_downgrade_wake\n\t"
 235                 "  popl      %%edx\n\t"
 236                 "  popl      %%ecx\n\t"
 237                 "  jmp       1b\n"
 238                 LOCK_SECTION_END
 239                 "# ending __downgrade_write\n"
 240                 : "=m"(sem->count)
 241                 : "a"(sem), "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
 242                 : "memory", "cc");
 243 }
 244
 245 /*
 246  * implement atomic add functionality
 247  */
 248 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 249 {
 250         __asm__ __volatile__(
 251 LOCK_PREFIX     "addl %1,%0"
 252                 : "=m"(sem->count)
 253                 : "ir"(delta), "m"(sem->count));
 254 }
 255
 256 /*
 257  * implement exchange and add functionality
 258  */
 259 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 260 {
 261         int tmp = delta;
 262
 263         __asm__ __volatile__(
 264 LOCK_PREFIX     "xadd %0,(%2)"
 265                 : "+r"(tmp), "=m"(sem->count)
 266                 : "r"(sem), "m"(sem->count)
 267                 : "memory");
 268
 269         return tmp+delta;
 270 }
 271
 272 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 273 {
 274         return (sem->count != 0);
 275 }
 276
 277 #endif /* __KERNEL__ */
 278 #endif /* _I386_RWSEM_H */