arch/x86/include/asm/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-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 #include <asm/asm.h>
  45
  46 struct rwsem_waiter;
  47
  48 extern asmregparm struct rw_semaphore *
  49  rwsem_down_read_failed(struct rw_semaphore *sem);
  50 extern asmregparm struct rw_semaphore *
  51  rwsem_down_write_failed(struct rw_semaphore *sem);
  52 extern asmregparm struct rw_semaphore *
  53  rwsem_wake(struct rw_semaphore *);
  54 extern asmregparm struct rw_semaphore *
  55  rwsem_downgrade_wake(struct rw_semaphore *sem);
  56
  57 /*
  58  * the semaphore definition
  59  *
  60  * The bias values and the counter type limits the number of
  61  * potential readers/writers to 32767 for 32 bits and 2147483647
  62  * for 64 bits.
  63  */
  64
  65 #ifdef CONFIG_X86_64
  66 # define RWSEM_ACTIVE_MASK              0xffffffffL
  67 #else
  68 # define RWSEM_ACTIVE_MASK              0x0000ffffL
  69 #endif
  70
  71 #define RWSEM_UNLOCKED_VALUE            0x00000000L
  72 #define RWSEM_ACTIVE_BIAS               0x00000001L
  73 #define RWSEM_WAITING_BIAS              (-RWSEM_ACTIVE_MASK-1)
  74 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  75 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  76
  77 typedef signed long rwsem_count_t;
  78
  79 struct rw_semaphore {
  80         rwsem_count_t           count;
  81         spinlock_t              wait_lock;
  82         struct list_head        wait_list;
  83 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  84         struct lockdep_map dep_map;
  85 #endif
  86 };
  87
  88 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  89 # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  90 #else
  91 # define __RWSEM_DEP_MAP_INIT(lockname)
  92 #endif
  93
  94
  95 #define __RWSEM_INITIALIZER(name)                               \
  96 {                                                               \
  97         RWSEM_UNLOCKED_VALUE, __SPIN_LOCK_UNLOCKED((name).wait_lock), \
  98         LIST_HEAD_INIT((name).wait_list) __RWSEM_DEP_MAP_INIT(name) \
  99 }
 100
 101 #define DECLARE_RWSEM(name)                                     \
 102         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
 103
 104 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 105                          struct lock_class_key *key);
 106
 107 #define init_rwsem(sem)                                         \
 108 do {                                                            \
 109         static struct lock_class_key __key;                     \
 110                                                                 \
 111         __init_rwsem((sem), #sem, &__key);                      \
 112 } while (0)
 113
 114 /*
 115  * lock for reading
 116  */
 117 static inline void __down_read(struct rw_semaphore *sem)
 118 {
 119         asm volatile("# beginning down_read\n\t"
 120                      LOCK_PREFIX _ASM_INC "(%1)\n\t"
 121                      /* adds 0x00000001, returns the old value */
 122                      "  jns        1f\n"
 123                      "  call call_rwsem_down_read_failed\n"
 124                      "1:\n\t"
 125                      "# ending down_read\n\t"
 126                      : "+m" (sem->count)
 127                      : "a" (sem)
 128                      : "memory", "cc");
 129 }
 130
 131 /*
 132  * trylock for reading -- returns 1 if successful, 0 if contention
 133  */
 134 static inline int __down_read_trylock(struct rw_semaphore *sem)
 135 {
 136         rwsem_count_t result, tmp;
 137         asm volatile("# beginning __down_read_trylock\n\t"
 138                      "  mov          %0,%1\n\t"
 139                      "1:\n\t"
 140                      "  mov          %1,%2\n\t"
 141                      "  add          %3,%2\n\t"
 142                      "  jle          2f\n\t"
 143                      LOCK_PREFIX "  cmpxchg  %2,%0\n\t"
 144                      "  jnz          1b\n\t"
 145                      "2:\n\t"
 146                      "# ending __down_read_trylock\n\t"
 147                      : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
 148                      : "i" (RWSEM_ACTIVE_READ_BIAS)
 149                      : "memory", "cc");
 150         return result >= 0 ? 1 : 0;
 151 }
 152
 153 /*
 154  * lock for writing
 155  */
 156 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 157 {
 158         rwsem_count_t tmp;
 159
 160         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 161         asm volatile("# beginning down_write\n\t"
 162                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 163                      /* subtract 0x0000ffff, returns the old value */
 164                      "  test      %1,%1\n\t"
 165                      /* was the count 0 before? */
 166                      "  jz        1f\n"
 167                      "  call call_rwsem_down_write_failed\n"
 168                      "1:\n"
 169                      "# ending down_write"
 170                      : "+m" (sem->count), "=d" (tmp)
 171                      : "a" (sem), "1" (tmp)
 172                      : "memory", "cc");
 173 }
 174
 175 static inline void __down_write(struct rw_semaphore *sem)
 176 {
 177         __down_write_nested(sem, 0);
 178 }
 179
 180 /*
 181  * trylock for writing -- returns 1 if successful, 0 if contention
 182  */
 183 static inline int __down_write_trylock(struct rw_semaphore *sem)
 184 {
 185         rwsem_count_t ret = cmpxchg(&sem->count,
 186                                     RWSEM_UNLOCKED_VALUE,
 187                                     RWSEM_ACTIVE_WRITE_BIAS);
 188         if (ret == RWSEM_UNLOCKED_VALUE)
 189                 return 1;
 190         return 0;
 191 }
 192
 193 /*
 194  * unlock after reading
 195  */
 196 static inline void __up_read(struct rw_semaphore *sem)
 197 {
 198         rwsem_count_t tmp = -RWSEM_ACTIVE_READ_BIAS;
 199         asm volatile("# beginning __up_read\n\t"
 200                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 201                      /* subtracts 1, returns the old value */
 202                      "  jns        1f\n\t"
 203                      "  call call_rwsem_wake\n"
 204                      "1:\n"
 205                      "# ending __up_read\n"
 206                      : "+m" (sem->count), "=d" (tmp)
 207                      : "a" (sem), "1" (tmp)
 208                      : "memory", "cc");
 209 }
 210
 211 /*
 212  * unlock after writing
 213  */
 214 static inline void __up_write(struct rw_semaphore *sem)
 215 {
 216         rwsem_count_t tmp;
 217         asm volatile("# beginning __up_write\n\t"
 218                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 219                      /* tries to transition
 220                         0xffff0001 -> 0x00000000 */
 221                      "  jz       1f\n"
 222                      "  call call_rwsem_wake\n"
 223                      "1:\n\t"
 224                      "# ending __up_write\n"
 225                      : "+m" (sem->count), "=d" (tmp)
 226                      : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
 227                      : "memory", "cc");
 228 }
 229
 230 /*
 231  * downgrade write lock to read lock
 232  */
 233 static inline void __downgrade_write(struct rw_semaphore *sem)
 234 {
 235         asm volatile("# beginning __downgrade_write\n\t"
 236                      LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
 237                      /*
 238                       * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
 239                       *     0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
 240                       */
 241                      "  jns       1f\n\t"
 242                      "  call call_rwsem_downgrade_wake\n"
 243                      "1:\n\t"
 244                      "# ending __downgrade_write\n"
 245                      : "+m" (sem->count)
 246                      : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
 247                      : "memory", "cc");
 248 }
 249
 250 /*
 251  * implement atomic add functionality
 252  */
 253 static inline void rwsem_atomic_add(rwsem_count_t delta,
 254                                     struct rw_semaphore *sem)
 255 {
 256         asm volatile(LOCK_PREFIX _ASM_ADD "%1,%0"
 257                      : "+m" (sem->count)
 258                      : "er" (delta));
 259 }
 260
 261 /*
 262  * implement exchange and add functionality
 263  */
 264 static inline rwsem_count_t rwsem_atomic_update(rwsem_count_t delta,
 265                                                 struct rw_semaphore *sem)
 266 {
 267         rwsem_count_t tmp = delta;
 268
 269         asm volatile(LOCK_PREFIX "xadd %0,%1"
 270                      : "+r" (tmp), "+m" (sem->count)
 271                      : : "memory");
 272
 273         return tmp + delta;
 274 }
 275
 276 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 277 {
 278         return (sem->count != 0);
 279 }
 280
 281 #endif /* __KERNEL__ */
 282 #endif /* _ASM_X86_RWSEM_H */