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  * Trylock by Brian Watson (Brian.J.Watson@compaq.com).
   8  *
   9  *
  10  * The MSW of the count is the negated number of active writers and waiting
  11  * lockers, and the LSW is the total number of active locks
  12  *
  13  * The lock count is initialized to 0 (no active and no waiting lockers).
  14  *
  15  * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
  16  * uncontended lock. This can be determined because XADD returns the old value.
  17  * Readers increment by 1 and see a positive value when uncontended, negative
  18  * if there are writers (and maybe) readers waiting (in which case it goes to
  19  * sleep).
  20  *
  21  * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
  22  * be extended to 65534 by manually checking the whole MSW rather than relying
  23  * on the S flag.
  24  *
  25  * The value of ACTIVE_BIAS supports up to 65535 active processes.
  26  *
  27  * This should be totally fair - if anything is waiting, a process that wants a
  28  * lock will go to the back of the queue. When the currently active lock is
  29  * released, if there's a writer at the front of the queue, then that and only
  30  * that will be woken up; if there's a bunch of consequtive readers at the
  31  * front, then they'll all be woken up, but no other readers will be.
  32  */
  33
  34 #ifndef _I386_RWSEM_H
  35 #define _I386_RWSEM_H
  36
  37 #ifndef _LINUX_RWSEM_H
  38 #error please dont include asm/rwsem.h directly, use linux/rwsem.h instead
  39 #endif
  40
  41 #ifdef __KERNEL__
  42
  43 #include <linux/list.h>
  44 #include <linux/spinlock.h>
  45
  46 struct rwsem_waiter;
  47
  48 extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
  49 extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
  50 extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
  51
  52 /*
  53  * the semaphore definition
  54  */
  55 struct rw_semaphore {
  56         signed long             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      (%%eax)\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                 "  pushl     %%ecx\n\t"
 109                 "  pushl     %%edx\n\t"
 110                 "  call      rwsem_down_read_failed\n\t"
 111                 "  popl      %%edx\n\t"
 112                 "  popl      %%ecx\n\t"
 113                 "  jmp       1b\n"
 114                 LOCK_SECTION_END
 115                 "# ending down_read\n\t"
 116                 : "=m"(sem->count)
 117                 : "a"(sem), "m"(sem->count)
 118                 : "memory", "cc");
 119 }
 120
 121 /*
 122  * trylock for reading -- returns 1 if successful, 0 if contention
 123  */
 124 static inline int __down_read_trylock(struct rw_semaphore *sem)
 125 {
 126         __s32 result, tmp;
 127         __asm__ __volatile__(
 128                 "# beginning __down_read_trylock\n\t"
 129                 "  movl      %0,%1\n\t"
 130                 "1:\n\t"
 131                 "  movl      %1,%2\n\t"
 132                 "  addl      %3,%2\n\t"
 133                 "  jle       2f\n\t"
 134 LOCK_PREFIX     "  cmpxchgl  %2,%0\n\t"
 135                 "  jnz       1b\n\t"
 136                 "2:\n\t"
 137                 "# ending __down_read_trylock\n\t"
 138                 : "+m"(sem->count), "=&a"(result), "=&r"(tmp)
 139                 : "i"(RWSEM_ACTIVE_READ_BIAS)
 140                 : "memory", "cc");
 141         return result>=0 ? 1 : 0;
 142 }
 143
 144 /*
 145  * lock for writing
 146  */
 147 static inline void __down_write(struct rw_semaphore *sem)
 148 {
 149         int tmp;
 150
 151         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 152         __asm__ __volatile__(
 153                 "# beginning down_write\n\t"
 154 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
 155                 "  testl     %%edx,%%edx\n\t" /* was the count 0 before? */
 156                 "  jnz       2f\n\t" /* jump if we weren't granted the lock */
 157                 "1:\n\t"
 158                 LOCK_SECTION_START("")
 159                 "2:\n\t"
 160                 "  pushl     %%ecx\n\t"
 161                 "  call      rwsem_down_write_failed\n\t"
 162                 "  popl      %%ecx\n\t"
 163                 "  jmp       1b\n"
 164                 LOCK_SECTION_END
 165                 "# ending down_write"
 166                 : "=m"(sem->count), "=d"(tmp)
 167                 : "a"(sem), "1"(tmp), "m"(sem->count)
 168                 : "memory", "cc");
 169 }
 170
 171 /*
 172  * trylock for writing -- returns 1 if successful, 0 if contention
 173  */
 174 static inline int __down_write_trylock(struct rw_semaphore *sem)
 175 {
 176         signed long ret = cmpxchg(&sem->count,
 177                                   RWSEM_UNLOCKED_VALUE,
 178                                   RWSEM_ACTIVE_WRITE_BIAS);
 179         if (ret == RWSEM_UNLOCKED_VALUE)
 180                 return 1;
 181         return 0;
 182 }
 183
 184 /*
 185  * unlock after reading
 186  */
 187 static inline void __up_read(struct rw_semaphore *sem)
 188 {
 189         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 190         __asm__ __volatile__(
 191                 "# beginning __up_read\n\t"
 192 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
 193                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 194                 "1:\n\t"
 195                 LOCK_SECTION_START("")
 196                 "2:\n\t"
 197                 "  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
 198                 "  jnz       1b\n\t"
 199                 "  pushl     %%ecx\n\t"
 200                 "  call      rwsem_wake\n\t"
 201                 "  popl      %%ecx\n\t"
 202                 "  jmp       1b\n"
 203                 LOCK_SECTION_END
 204                 "# ending __up_read\n"
 205                 : "=m"(sem->count), "=d"(tmp)
 206                 : "a"(sem), "1"(tmp), "m"(sem->count)
 207                 : "memory", "cc");
 208 }
 209
 210 /*
 211  * unlock after writing
 212  */
 213 static inline void __up_write(struct rw_semaphore *sem)
 214 {
 215         __asm__ __volatile__(
 216                 "# beginning __up_write\n\t"
 217                 "  movl      %2,%%edx\n\t"
 218 LOCK_PREFIX     "  xaddl     %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
 219                 "  jnz       2f\n\t" /* jump if the lock is being waited upon */
 220                 "1:\n\t"
 221                 LOCK_SECTION_START("")
 222                 "2:\n\t"
 223                 "  decw      %%dx\n\t" /* did the active count reduce to 0? */
 224                 "  jnz       1b\n\t" /* jump back if not */
 225                 "  pushl     %%ecx\n\t"
 226                 "  call      rwsem_wake\n\t"
 227                 "  popl      %%ecx\n\t"
 228                 "  jmp       1b\n"
 229                 LOCK_SECTION_END
 230                 "# ending __up_write\n"
 231                 : "=m"(sem->count)
 232                 : "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS), "m"(sem->count)
 233                 : "memory", "cc", "edx");
 234 }
 235
 236 /*
 237  * implement atomic add functionality
 238  */
 239 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 240 {
 241         __asm__ __volatile__(
 242 LOCK_PREFIX     "addl %1,%0"
 243                 :"=m"(sem->count)
 244                 :"ir"(delta), "m"(sem->count));
 245 }
 246
 247 /*
 248  * implement exchange and add functionality
 249  */
 250 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 251 {
 252         int tmp = delta;
 253
 254         __asm__ __volatile__(
 255 LOCK_PREFIX     "xadd %0,(%2)"
 256                 : "+r"(tmp), "=m"(sem->count)
 257                 : "r"(sem), "m"(sem->count)
 258                 : "memory");
 259
 260         return tmp+delta;
 261 }
 262
 263 #endif /* __KERNEL__ */
 264 #endif /* _I386_RWSEM_H */