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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-x86/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 _ASM_X86_RWSEM_H
  33 #define _ASM_X86_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 asmregparm struct rw_semaphore *
  48  rwsem_down_read_failed(struct rw_semaphore *sem);
  49 extern asmregparm struct rw_semaphore *
  50  rwsem_down_write_failed(struct rw_semaphore *sem);
  51 extern asmregparm struct rw_semaphore *
  52  rwsem_wake(struct rw_semaphore *);
  53 extern asmregparm struct rw_semaphore *
  54  rwsem_downgrade_wake(struct rw_semaphore *sem);
  55
  56 /*
  57  * the semaphore definition
  58  */
  59
  60 #define RWSEM_UNLOCKED_VALUE            0x00000000
  61 #define RWSEM_ACTIVE_BIAS               0x00000001
  62 #define RWSEM_ACTIVE_MASK               0x0000ffff
  63 #define RWSEM_WAITING_BIAS              (-0x00010000)
  64 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  65 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  66
  67 struct rw_semaphore {
  68         signed long             count;
  69         spinlock_t              wait_lock;
  70         struct list_head        wait_list;
  71 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  72         struct lockdep_map dep_map;
  73 #endif
  74 };
  75
  76 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  77 # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  78 #else
  79 # define __RWSEM_DEP_MAP_INIT(lockname)
  80 #endif
  81
  82
  83 #define __RWSEM_INITIALIZER(name)                               \
  84 {                                                               \
  85         RWSEM_UNLOCKED_VALUE, __SPIN_LOCK_UNLOCKED((name).wait_lock), \
  86         LIST_HEAD_INIT((name).wait_list) __RWSEM_DEP_MAP_INIT(name) \
  87 }
  88
  89 #define DECLARE_RWSEM(name)                                     \
  90         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  91
  92 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
  93                          struct lock_class_key *key);
  94
  95 #define init_rwsem(sem)                                         \
  96 do {                                                            \
  97         static struct lock_class_key __key;                     \
  98                                                                 \
  99         __init_rwsem((sem), #sem, &__key);                      \
 100 } while (0)
 101
 102 /*
 103  * lock for reading
 104  */
 105 static inline void __down_read(struct rw_semaphore *sem)
 106 {
 107         asm volatile("# beginning down_read\n\t"
 108                      LOCK_PREFIX "  incl      (%%eax)\n\t"
 109                      /* adds 0x00000001, returns the old value */
 110                      "  jns        1f\n"
 111                      "  call call_rwsem_down_read_failed\n"
 112                      "1:\n\t"
 113                      "# ending down_read\n\t"
 114                      : "+m" (sem->count)
 115                      : "a" (sem)
 116                      : "memory", "cc");
 117 }
 118
 119 /*
 120  * trylock for reading -- returns 1 if successful, 0 if contention
 121  */
 122 static inline int __down_read_trylock(struct rw_semaphore *sem)
 123 {
 124         __s32 result, tmp;
 125         asm volatile("# 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  * lock for writing
 143  */
 144 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 145 {
 146         int tmp;
 147
 148         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 149         asm volatile("# beginning down_write\n\t"
 150                      LOCK_PREFIX "  xadd      %%edx,(%%eax)\n\t"
 151                      /* subtract 0x0000ffff, returns the old value */
 152                      "  testl     %%edx,%%edx\n\t"
 153                      /* was the count 0 before? */
 154                      "  jz        1f\n"
 155                      "  call call_rwsem_down_write_failed\n"
 156                      "1:\n"
 157                      "# ending down_write"
 158                      : "+m" (sem->count), "=d" (tmp)
 159                      : "a" (sem), "1" (tmp)
 160                      : "memory", "cc");
 161 }
 162
 163 static inline void __down_write(struct rw_semaphore *sem)
 164 {
 165         __down_write_nested(sem, 0);
 166 }
 167
 168 /*
 169  * trylock for writing -- returns 1 if successful, 0 if contention
 170  */
 171 static inline int __down_write_trylock(struct rw_semaphore *sem)
 172 {
 173         signed long ret = cmpxchg(&sem->count,
 174                                   RWSEM_UNLOCKED_VALUE,
 175                                   RWSEM_ACTIVE_WRITE_BIAS);
 176         if (ret == RWSEM_UNLOCKED_VALUE)
 177                 return 1;
 178         return 0;
 179 }
 180
 181 /*
 182  * unlock after reading
 183  */
 184 static inline void __up_read(struct rw_semaphore *sem)
 185 {
 186         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 187         asm volatile("# beginning __up_read\n\t"
 188                      LOCK_PREFIX "  xadd      %%edx,(%%eax)\n\t"
 189                      /* subtracts 1, returns the old value */
 190                      "  jns        1f\n\t"
 191                      "  call call_rwsem_wake\n"
 192                      "1:\n"
 193                      "# ending __up_read\n"
 194                      : "+m" (sem->count), "=d" (tmp)
 195                      : "a" (sem), "1" (tmp)
 196                      : "memory", "cc");
 197 }
 198
 199 /*
 200  * unlock after writing
 201  */
 202 static inline void __up_write(struct rw_semaphore *sem)
 203 {
 204         asm volatile("# beginning __up_write\n\t"
 205                      "  movl      %2,%%edx\n\t"
 206                      LOCK_PREFIX "  xaddl     %%edx,(%%eax)\n\t"
 207                      /* tries to transition
 208                         0xffff0001 -> 0x00000000 */
 209                      "  jz       1f\n"
 210                      "  call call_rwsem_wake\n"
 211                      "1:\n\t"
 212                      "# ending __up_write\n"
 213                      : "+m" (sem->count)
 214                      : "a" (sem), "i" (-RWSEM_ACTIVE_WRITE_BIAS)
 215                      : "memory", "cc", "edx");
 216 }
 217
 218 /*
 219  * downgrade write lock to read lock
 220  */
 221 static inline void __downgrade_write(struct rw_semaphore *sem)
 222 {
 223         asm volatile("# beginning __downgrade_write\n\t"
 224                      LOCK_PREFIX "  addl      %2,(%%eax)\n\t"
 225                      /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
 226                      "  jns       1f\n\t"
 227                      "  call call_rwsem_downgrade_wake\n"
 228                      "1:\n\t"
 229                      "# ending __downgrade_write\n"
 230                      : "+m" (sem->count)
 231                      : "a" (sem), "i" (-RWSEM_WAITING_BIAS)
 232                      : "memory", "cc");
 233 }
 234
 235 /*
 236  * implement atomic add functionality
 237  */
 238 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 239 {
 240         asm volatile(LOCK_PREFIX "addl %1,%0"
 241                      : "+m" (sem->count)
 242                      : "ir" (delta));
 243 }
 244
 245 /*
 246  * implement exchange and add functionality
 247  */
 248 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 249 {
 250         int tmp = delta;
 251
 252         asm volatile(LOCK_PREFIX "xadd %0,%1"
 253                      : "+r" (tmp), "+m" (sem->count)
 254                      : : "memory");
 255
 256         return tmp + delta;
 257 }
 258
 259 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 260 {
 261         return (sem->count != 0);
 262 }
 263
 264 #endif /* __KERNEL__ */
 265 #endif /* _ASM_X86_RWSEM_H */