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 consecutive 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 #include <asm/asm.h>
  41
  42 /*
  43  * The bias values and the counter type limits the number of
  44  * potential readers/writers to 32767 for 32 bits and 2147483647
  45  * for 64 bits.
  46  */
  47
  48 #ifdef CONFIG_X86_64
  49 # define RWSEM_ACTIVE_MASK              0xffffffffL
  50 #else
  51 # define RWSEM_ACTIVE_MASK              0x0000ffffL
  52 #endif
  53
  54 #define RWSEM_UNLOCKED_VALUE            0x00000000L
  55 #define RWSEM_ACTIVE_BIAS               0x00000001L
  56 #define RWSEM_WAITING_BIAS              (-RWSEM_ACTIVE_MASK-1)
  57 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  58 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  59
  60 /*
  61  * lock for reading
  62  */
  63 static inline void __down_read(struct rw_semaphore *sem)
  64 {
  65         asm volatile("# beginning down_read\n\t"
  66                      LOCK_PREFIX _ASM_INC "(%1)\n\t"
  67                      /* adds 0x00000001 */
  68                      "  jns        1f\n"
  69                      "  call call_rwsem_down_read_failed\n"
  70                      "1:\n\t"
  71                      "# ending down_read\n\t"
  72                      : "+m" (sem->count)
  73                      : "a" (sem)
  74                      : "memory", "cc");
  75 }
  76
  77 /*
  78  * trylock for reading -- returns 1 if successful, 0 if contention
  79  */
  80 static inline bool __down_read_trylock(struct rw_semaphore *sem)
  81 {
  82         long result, tmp;
  83         asm volatile("# beginning __down_read_trylock\n\t"
  84                      "  mov          %0,%1\n\t"
  85                      "1:\n\t"
  86                      "  mov          %1,%2\n\t"
  87                      "  add          %3,%2\n\t"
  88                      "  jle          2f\n\t"
  89                      LOCK_PREFIX "  cmpxchg  %2,%0\n\t"
  90                      "  jnz          1b\n\t"
  91                      "2:\n\t"
  92                      "# ending __down_read_trylock\n\t"
  93                      : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
  94                      : "i" (RWSEM_ACTIVE_READ_BIAS)
  95                      : "memory", "cc");
  96         return result >= 0;
  97 }
  98
  99 /*
 100  * lock for writing
 101  */
 102 #define ____down_write(sem, slow_path)                  \
 103 ({                                                      \
 104         long tmp;                                       \
 105         struct rw_semaphore* ret;                       \
 106                                                         \
 107         asm volatile("# beginning down_write\n\t"       \
 108                      LOCK_PREFIX "  xadd      %1,(%4)\n\t"      \
 109                      /* adds 0xffff0001, returns the old value */ \
 110                      "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
 111                      /* was the active mask 0 before? */\
 112                      "  jz        1f\n"                 \
 113                      "  call " slow_path "\n"           \
 114                      "1:\n"                             \
 115                      "# ending down_write"              \
 116                      : "+m" (sem->count), "=d" (tmp),   \
 117                        "=a" (ret), ASM_CALL_CONSTRAINT  \
 118                      : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
 119                      : "memory", "cc");                 \
 120         ret;                                            \
 121 })
 122
 123 static inline void __down_write(struct rw_semaphore *sem)
 124 {
 125         ____down_write(sem, "call_rwsem_down_write_failed");
 126 }
 127
 128 static inline int __down_write_killable(struct rw_semaphore *sem)
 129 {
 130         if (IS_ERR(____down_write(sem, "call_rwsem_down_write_failed_killable")))
 131                 return -EINTR;
 132
 133         return 0;
 134 }
 135
 136 /*
 137  * trylock for writing -- returns 1 if successful, 0 if contention
 138  */
 139 static inline bool __down_write_trylock(struct rw_semaphore *sem)
 140 {
 141         bool result;
 142         long tmp0, tmp1;
 143         asm volatile("# beginning __down_write_trylock\n\t"
 144                      "  mov          %0,%1\n\t"
 145                      "1:\n\t"
 146                      "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
 147                      /* was the active mask 0 before? */
 148                      "  jnz          2f\n\t"
 149                      "  mov          %1,%2\n\t"
 150                      "  add          %4,%2\n\t"
 151                      LOCK_PREFIX "  cmpxchg  %2,%0\n\t"
 152                      "  jnz          1b\n\t"
 153                      "2:\n\t"
 154                      CC_SET(e)
 155                      "# ending __down_write_trylock\n\t"
 156                      : "+m" (sem->count), "=&a" (tmp0), "=&r" (tmp1),
 157                        CC_OUT(e) (result)
 158                      : "er" (RWSEM_ACTIVE_WRITE_BIAS)
 159                      : "memory");
 160         return result;
 161 }
 162
 163 /*
 164  * unlock after reading
 165  */
 166 static inline void __up_read(struct rw_semaphore *sem)
 167 {
 168         long tmp;
 169         asm volatile("# beginning __up_read\n\t"
 170                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 171                      /* subtracts 1, returns the old value */
 172                      "  jns        1f\n\t"
 173                      "  call call_rwsem_wake\n" /* expects old value in %edx */
 174                      "1:\n"
 175                      "# ending __up_read\n"
 176                      : "+m" (sem->count), "=d" (tmp)
 177                      : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)
 178                      : "memory", "cc");
 179 }
 180
 181 /*
 182  * unlock after writing
 183  */
 184 static inline void __up_write(struct rw_semaphore *sem)
 185 {
 186         long tmp;
 187         asm volatile("# beginning __up_write\n\t"
 188                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
 189                      /* subtracts 0xffff0001, returns the old value */
 190                      "  jns        1f\n\t"
 191                      "  call call_rwsem_wake\n" /* expects old value in %edx */
 192                      "1:\n\t"
 193                      "# ending __up_write\n"
 194                      : "+m" (sem->count), "=d" (tmp)
 195                      : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
 196                      : "memory", "cc");
 197 }
 198
 199 /*
 200  * downgrade write lock to read lock
 201  */
 202 static inline void __downgrade_write(struct rw_semaphore *sem)
 203 {
 204         asm volatile("# beginning __downgrade_write\n\t"
 205                      LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
 206                      /*
 207                       * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
 208                       *     0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
 209                       */
 210                      "  jns       1f\n\t"
 211                      "  call call_rwsem_downgrade_wake\n"
 212                      "1:\n\t"
 213                      "# ending __downgrade_write\n"
 214                      : "+m" (sem->count)
 215                      : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
 216                      : "memory", "cc");
 217 }
 218
 219 #endif /* __KERNEL__ */
 220 #endif /* _ASM_X86_RWSEM_H */