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   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 #if RWSEM_DEBUG
  65         int                     debug;
  66 #endif
  67 };
  68
  69 /*
  70  * initialisation
  71  */
  72 #if RWSEM_DEBUG
  73 #define __RWSEM_DEBUG_INIT      , 0
  74 #else
  75 #define __RWSEM_DEBUG_INIT      /* */
  76 #endif
  77
  78 #define __RWSEM_INITIALIZER(name) \
  79 { RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
  80         __RWSEM_DEBUG_INIT }
  81
  82 #define DECLARE_RWSEM(name) \
  83         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  84
  85 static inline void init_rwsem(struct rw_semaphore *sem)
  86 {
  87         sem->count = RWSEM_UNLOCKED_VALUE;
  88         spin_lock_init(&sem->wait_lock);
  89         INIT_LIST_HEAD(&sem->wait_list);
  90 #if RWSEM_DEBUG
  91         sem->debug = 0;
  92 #endif
  93 }
  94
  95 /*
  96  * lock for reading
  97  */
  98 static inline void __down_read(struct rw_semaphore *sem)
  99 {
 100         __asm__ __volatile__(
 101                 "# beginning down_read\n\t"
 102 LOCK_PREFIX     "  incl      (%%eax)\n\t" /* adds 0x00000001, returns the old value */
 103                 "  js        2f\n\t" /* jump if we weren't granted the lock */
 104                 "1:\n\t"
 105                 LOCK_SECTION_START("")
 106                 "2:\n\t"
 107                 "  pushl     %%ecx\n\t"
 108                 "  pushl     %%edx\n\t"
 109                 "  call      rwsem_down_read_failed\n\t"
 110                 "  popl      %%edx\n\t"
 111                 "  popl      %%ecx\n\t"
 112                 "  jmp       1b\n"
 113                 LOCK_SECTION_END
 114                 "# ending down_read\n\t"
 115                 : "=m"(sem->count)
 116                 : "a"(sem), "m"(sem->count)
 117                 : "memory", "cc");
 118 }
 119
 120 /*
 121  * trylock for reading -- returns 1 if successful, 0 if contention
 122  */
 123 static inline int __down_read_trylock(struct rw_semaphore *sem)
 124 {
 125         __s32 result, tmp;
 126         __asm__ __volatile__(
 127                 "# beginning __down_read_trylock\n\t"
 128                 "  movl      %0,%1\n\t"
 129                 "1:\n\t"
 130                 "  movl      %1,%2\n\t"
 131                 "  addl      %3,%2\n\t"
 132                 "  jle       2f\n\t"
 133 LOCK_PREFIX     "  cmpxchgl  %2,%0\n\t"
 134                 "  jnz       1b\n\t"
 135                 "2:\n\t"
 136                 "# ending __down_read_trylock\n\t"
 137                 : "+m"(sem->count), "=&a"(result), "=&r"(tmp)
 138                 : "i"(RWSEM_ACTIVE_READ_BIAS)
 139                 : "memory", "cc");
 140         return result>=0 ? 1 : 0;
 141 }
 142
 143 /*
 144  * lock for writing
 145  */
 146 static inline void __down_write(struct rw_semaphore *sem)
 147 {
 148         int tmp;
 149
 150         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 151         __asm__ __volatile__(
 152                 "# beginning down_write\n\t"
 153 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
 154                 "  testl     %%edx,%%edx\n\t" /* was the count 0 before? */
 155                 "  jnz       2f\n\t" /* jump if we weren't granted the lock */
 156                 "1:\n\t"
 157                 LOCK_SECTION_START("")
 158                 "2:\n\t"
 159                 "  pushl     %%ecx\n\t"
 160                 "  call      rwsem_down_write_failed\n\t"
 161                 "  popl      %%ecx\n\t"
 162                 "  jmp       1b\n"
 163                 LOCK_SECTION_END
 164                 "# ending down_write"
 165                 : "=m"(sem->count), "=d"(tmp)
 166                 : "a"(sem), "1"(tmp), "m"(sem->count)
 167                 : "memory", "cc");
 168 }
 169
 170 /*
 171  * trylock for writing -- returns 1 if successful, 0 if contention
 172  */
 173 static inline int __down_write_trylock(struct rw_semaphore *sem)
 174 {
 175         signed long ret = cmpxchg(&sem->count,
 176                                   RWSEM_UNLOCKED_VALUE,
 177                                   RWSEM_ACTIVE_WRITE_BIAS);
 178         if (ret == RWSEM_UNLOCKED_VALUE)
 179                 return 1;
 180         return 0;
 181 }
 182
 183 /*
 184  * unlock after reading
 185  */
 186 static inline void __up_read(struct rw_semaphore *sem)
 187 {
 188         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 189         __asm__ __volatile__(
 190                 "# beginning __up_read\n\t"
 191 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
 192                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 193                 "1:\n\t"
 194                 LOCK_SECTION_START("")
 195                 "2:\n\t"
 196                 "  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
 197                 "  jnz       1b\n\t"
 198                 "  pushl     %%ecx\n\t"
 199                 "  call      rwsem_wake\n\t"
 200                 "  popl      %%ecx\n\t"
 201                 "  jmp       1b\n"
 202                 LOCK_SECTION_END
 203                 "# ending __up_read\n"
 204                 : "=m"(sem->count), "=d"(tmp)
 205                 : "a"(sem), "1"(tmp), "m"(sem->count)
 206                 : "memory", "cc");
 207 }
 208
 209 /*
 210  * unlock after writing
 211  */
 212 static inline void __up_write(struct rw_semaphore *sem)
 213 {
 214         __asm__ __volatile__(
 215                 "# beginning __up_write\n\t"
 216                 "  movl      %2,%%edx\n\t"
 217 LOCK_PREFIX     "  xaddl     %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
 218                 "  jnz       2f\n\t" /* jump if the lock is being waited upon */
 219                 "1:\n\t"
 220                 LOCK_SECTION_START("")
 221                 "2:\n\t"
 222                 "  decw      %%dx\n\t" /* did the active count reduce to 0? */
 223                 "  jnz       1b\n\t" /* jump back if not */
 224                 "  pushl     %%ecx\n\t"
 225                 "  call      rwsem_wake\n\t"
 226                 "  popl      %%ecx\n\t"
 227                 "  jmp       1b\n"
 228                 LOCK_SECTION_END
 229                 "# ending __up_write\n"
 230                 : "=m"(sem->count)
 231                 : "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS), "m"(sem->count)
 232                 : "memory", "cc", "edx");
 233 }
 234
 235 /*
 236  * downgrade write lock to read lock
 237  */
 238 static inline void __downgrade_write(struct rw_semaphore *sem)
 239 {
 240         __asm__ __volatile__(
 241                 "# beginning __downgrade_write\n\t"
 242 LOCK_PREFIX     "  addl      %2,(%%eax)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
 243                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 244                 "1:\n\t"
 245                 LOCK_SECTION_START("")
 246                 "2:\n\t"
 247                 "  pushl     %%ecx\n\t"
 248                 "  pushl     %%edx\n\t"
 249                 "  call      rwsem_downgrade_wake\n\t"
 250                 "  popl      %%edx\n\t"
 251                 "  popl      %%ecx\n\t"
 252                 "  jmp       1b\n"
 253                 LOCK_SECTION_END
 254                 "# ending __downgrade_write\n"
 255                 : "=m"(sem->count)
 256                 : "a"(sem), "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
 257                 : "memory", "cc");
 258 }
 259
 260 /*
 261  * implement atomic add functionality
 262  */
 263 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 264 {
 265         __asm__ __volatile__(
 266 LOCK_PREFIX     "addl %1,%0"
 267                 : "=m"(sem->count)
 268                 : "ir"(delta), "m"(sem->count));
 269 }
 270
 271 /*
 272  * implement exchange and add functionality
 273  */
 274 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 275 {
 276         int tmp = delta;
 277
 278         __asm__ __volatile__(
 279 LOCK_PREFIX     "xadd %0,(%2)"
 280                 : "+r"(tmp), "=m"(sem->count)
 281                 : "r"(sem), "m"(sem->count)
 282                 : "memory");
 283
 284         return tmp+delta;
 285 }
 286
 287 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 288 {
 289         return (sem->count != 0);
 290 }
 291
 292 #endif /* __KERNEL__ */
 293 #endif /* _I386_RWSEM_H */