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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 #include <linux/lockdep.h>
  44
  45 struct rwsem_waiter;
  46
  47 extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
  48 extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
  49 extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
  50 extern struct rw_semaphore *FASTCALL(rwsem_downgrade_wake(struct rw_semaphore *sem));
  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 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  66         struct lockdep_map dep_map;
  67 #endif
  68 };
  69
  70 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  71 # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  72 #else
  73 # define __RWSEM_DEP_MAP_INIT(lockname)
  74 #endif
  75
  76
  77 #define __RWSEM_INITIALIZER(name) \
  78 { RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
  79         __RWSEM_DEP_MAP_INIT(name) }
  80
  81 #define DECLARE_RWSEM(name) \
  82         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  83
  84 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
  85                          struct lock_class_key *key);
  86
  87 #define init_rwsem(sem)                                         \
  88 do {                                                            \
  89         static struct lock_class_key __key;                     \
  90                                                                 \
  91         __init_rwsem((sem), #sem, &__key);                      \
  92 } while (0)
  93
  94 /*
  95  * lock for reading
  96  */
  97 static inline void __down_read(struct rw_semaphore *sem)
  98 {
  99         __asm__ __volatile__(
 100                 "# beginning down_read\n\t"
 101 LOCK_PREFIX     "  incl      (%%eax)\n\t" /* adds 0x00000001, returns the old value */
 102                 "  jns        1f\n"
 103                 "  call call_rwsem_down_read_failed\n"
 104                 "1:\n\t"
 105                 "# ending down_read\n\t"
 106                 : "+m" (sem->count)
 107                 : "a" (sem)
 108                 : "memory", "cc");
 109 }
 110
 111 /*
 112  * trylock for reading -- returns 1 if successful, 0 if contention
 113  */
 114 static inline int __down_read_trylock(struct rw_semaphore *sem)
 115 {
 116         __s32 result, tmp;
 117         __asm__ __volatile__(
 118                 "# beginning __down_read_trylock\n\t"
 119                 "  movl      %0,%1\n\t"
 120                 "1:\n\t"
 121                 "  movl      %1,%2\n\t"
 122                 "  addl      %3,%2\n\t"
 123                 "  jle       2f\n\t"
 124 LOCK_PREFIX     "  cmpxchgl  %2,%0\n\t"
 125                 "  jnz       1b\n\t"
 126                 "2:\n\t"
 127                 "# ending __down_read_trylock\n\t"
 128                 : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
 129                 : "i" (RWSEM_ACTIVE_READ_BIAS)
 130                 : "memory", "cc");
 131         return result>=0 ? 1 : 0;
 132 }
 133
 134 /*
 135  * lock for writing
 136  */
 137 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 138 {
 139         int tmp;
 140
 141         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 142         __asm__ __volatile__(
 143                 "# beginning down_write\n\t"
 144 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
 145                 "  testl     %%edx,%%edx\n\t" /* was the count 0 before? */
 146                 "  jz        1f\n"
 147                 "  call call_rwsem_down_write_failed\n"
 148                 "1:\n"
 149                 "# ending down_write"
 150                 : "+m" (sem->count), "=d" (tmp)
 151                 : "a" (sem), "1" (tmp)
 152                 : "memory", "cc");
 153 }
 154
 155 static inline void __down_write(struct rw_semaphore *sem)
 156 {
 157         __down_write_nested(sem, 0);
 158 }
 159
 160 /*
 161  * trylock for writing -- returns 1 if successful, 0 if contention
 162  */
 163 static inline int __down_write_trylock(struct rw_semaphore *sem)
 164 {
 165         signed long ret = cmpxchg(&sem->count,
 166                                   RWSEM_UNLOCKED_VALUE,
 167                                   RWSEM_ACTIVE_WRITE_BIAS);
 168         if (ret == RWSEM_UNLOCKED_VALUE)
 169                 return 1;
 170         return 0;
 171 }
 172
 173 /*
 174  * unlock after reading
 175  */
 176 static inline void __up_read(struct rw_semaphore *sem)
 177 {
 178         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 179         __asm__ __volatile__(
 180                 "# beginning __up_read\n\t"
 181 LOCK_PREFIX     "  xadd      %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
 182                 "  jns        1f\n\t"
 183                 "  call call_rwsem_wake\n"
 184                 "1:\n"
 185                 "# ending __up_read\n"
 186                 : "+m" (sem->count), "=d" (tmp)
 187                 : "a" (sem), "1" (tmp)
 188                 : "memory", "cc");
 189 }
 190
 191 /*
 192  * unlock after writing
 193  */
 194 static inline void __up_write(struct rw_semaphore *sem)
 195 {
 196         __asm__ __volatile__(
 197                 "# beginning __up_write\n\t"
 198                 "  movl      %2,%%edx\n\t"
 199 LOCK_PREFIX     "  xaddl     %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
 200                 "  jz       1f\n"
 201                 "  call call_rwsem_wake\n"
 202                 "1:\n\t"
 203                 "# ending __up_write\n"
 204                 : "+m" (sem->count)
 205                 : "a" (sem), "i" (-RWSEM_ACTIVE_WRITE_BIAS)
 206                 : "memory", "cc", "edx");
 207 }
 208
 209 /*
 210  * downgrade write lock to read lock
 211  */
 212 static inline void __downgrade_write(struct rw_semaphore *sem)
 213 {
 214         __asm__ __volatile__(
 215                 "# beginning __downgrade_write\n\t"
 216 LOCK_PREFIX     "  addl      %2,(%%eax)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
 217                 "  jns       1f\n\t"
 218                 "  call call_rwsem_downgrade_wake\n"
 219                 "1:\n\t"
 220                 "# ending __downgrade_write\n"
 221                 : "+m" (sem->count)
 222                 : "a" (sem), "i" (-RWSEM_WAITING_BIAS)
 223                 : "memory", "cc");
 224 }
 225
 226 /*
 227  * implement atomic add functionality
 228  */
 229 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 230 {
 231         __asm__ __volatile__(
 232 LOCK_PREFIX     "addl %1,%0"
 233                 : "+m" (sem->count)
 234                 : "ir" (delta));
 235 }
 236
 237 /*
 238  * implement exchange and add functionality
 239  */
 240 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 241 {
 242         int tmp = delta;
 243
 244         __asm__ __volatile__(
 245 LOCK_PREFIX     "xadd %0,%1"
 246                 : "+r" (tmp), "+m" (sem->count)
 247                 : : "memory");
 248
 249         return tmp+delta;
 250 }
 251
 252 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 253 {
 254         return (sem->count != 0);
 255 }
 256
 257 #endif /* __KERNEL__ */
 258 #endif /* _I386_RWSEM_H */