arch/x86/include/asm/rwsem.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 /* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
   3  *
   4  * Written by David Howells (dhowells@redhat.com).
   5  *
   6  * Derived from asm-x86/semaphore.h
   7  *
   8  *
   9  * The MSW of the count is the negated number of active writers and waiting
  10  * lockers, and the LSW is the total number of active locks
  11  *
  12  * The lock count is initialized to 0 (no active and no waiting lockers).
  13  *
  14  * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
  15  * uncontended lock. This can be determined because XADD returns the old value.
  16  * Readers increment by 1 and see a positive value when uncontended, negative
  17  * if there are writers (and maybe) readers waiting (in which case it goes to
  18  * sleep).
  19  *
  20  * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
  21  * be extended to 65534 by manually checking the whole MSW rather than relying
  22  * on the S flag.
  23  *
  24  * The value of ACTIVE_BIAS supports up to 65535 active processes.
  25  *
  26  * This should be totally fair - if anything is waiting, a process that wants a
  27  * lock will go to the back of the queue. When the currently active lock is
  28  * released, if there's a writer at the front of the queue, then that and only
  29  * that will be woken up; if there's a bunch of consecutive readers at the
  30  * front, then they'll all be woken up, but no other readers will be.
  31  */
  32
  33 #ifndef _ASM_X86_RWSEM_H
  34 #define _ASM_X86_RWSEM_H
  35
  36 #ifndef _LINUX_RWSEM_H
  37 #error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
  38 #endif
  39
  40 #ifdef __KERNEL__
  41 #include <asm/asm.h>
  42
  43 /*
  44  * The bias values and the counter type limits the number of
  45  * potential readers/writers to 32767 for 32 bits and 2147483647
  46  * for 64 bits.
  47  */
  48
  49 #ifdef CONFIG_X86_64
  50 # define RWSEM_ACTIVE_MASK              0xffffffffL
  51 #else
  52 # define RWSEM_ACTIVE_MASK              0x0000ffffL
  53 #endif
  54
  55 #define RWSEM_UNLOCKED_VALUE            0x00000000L
  56 #define RWSEM_ACTIVE_BIAS               0x00000001L
  57 #define RWSEM_WAITING_BIAS              (-RWSEM_ACTIVE_MASK-1)
  58 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  59 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  60
  61 /*
  62  * lock for reading
  63  */
  64 #define ____down_read(sem, slow_path)                                   \
  65 ({                                                                      \
  66         struct rw_semaphore* ret;                                       \
  67         asm volatile("# beginning down_read\n\t"                        \
  68                      LOCK_PREFIX _ASM_INC "(%[sem])\n\t"                \
  69                      /* adds 0x00000001 */                              \
  70                      "  jns        1f\n"                                \
  71                      "  call " slow_path "\n"                           \
  72                      "1:\n\t"                                           \
  73                      "# ending down_read\n\t"                           \
  74                      : "+m" (sem->count), "=a" (ret),                   \
  75                         ASM_CALL_CONSTRAINT                             \
  76                      : [sem] "a" (sem)                                  \
  77                      : "memory", "cc");                                 \
  78         ret;                                                            \
  79 })
  80
  81 static inline void __down_read(struct rw_semaphore *sem)
  82 {
  83         ____down_read(sem, "call_rwsem_down_read_failed");
  84 }
  85
  86 static inline int __down_read_killable(struct rw_semaphore *sem)
  87 {
  88         if (IS_ERR(____down_read(sem, "call_rwsem_down_read_failed_killable")))
  89                 return -EINTR;
  90         return 0;
  91 }
  92
  93 /*
  94  * trylock for reading -- returns 1 if successful, 0 if contention
  95  */
  96 static inline bool __down_read_trylock(struct rw_semaphore *sem)
  97 {
  98         long result, tmp;
  99         asm volatile("# beginning __down_read_trylock\n\t"
 100                      "  mov          %[count],%[result]\n\t"
 101                      "1:\n\t"
 102                      "  mov          %[result],%[tmp]\n\t"
 103                      "  add          %[inc],%[tmp]\n\t"
 104                      "  jle          2f\n\t"
 105                      LOCK_PREFIX "  cmpxchg  %[tmp],%[count]\n\t"
 106                      "  jnz          1b\n\t"
 107                      "2:\n\t"
 108                      "# ending __down_read_trylock\n\t"
 109                      : [count] "+m" (sem->count), [result] "=&a" (result),
 110                        [tmp] "=&r" (tmp)
 111                      : [inc] "i" (RWSEM_ACTIVE_READ_BIAS)
 112                      : "memory", "cc");
 113         return result >= 0;
 114 }
 115
 116 /*
 117  * lock for writing
 118  */
 119 #define ____down_write(sem, slow_path)                  \
 120 ({                                                      \
 121         long tmp;                                       \
 122         struct rw_semaphore* ret;                       \
 123                                                         \
 124         asm volatile("# beginning down_write\n\t"       \
 125                      LOCK_PREFIX "  xadd      %[tmp],(%[sem])\n\t"      \
 126                      /* adds 0xffff0001, returns the old value */ \
 127                      "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
 128                      /* was the active mask 0 before? */\
 129                      "  jz        1f\n"                 \
 130                      "  call " slow_path "\n"           \
 131                      "1:\n"                             \
 132                      "# ending down_write"              \
 133                      : "+m" (sem->count), [tmp] "=d" (tmp),     \
 134                        "=a" (ret), ASM_CALL_CONSTRAINT  \
 135                      : [sem] "a" (sem), "[tmp]" (RWSEM_ACTIVE_WRITE_BIAS) \
 136                      : "memory", "cc");                 \
 137         ret;                                            \
 138 })
 139
 140 static inline void __down_write(struct rw_semaphore *sem)
 141 {
 142         ____down_write(sem, "call_rwsem_down_write_failed");
 143 }
 144
 145 static inline int __down_write_killable(struct rw_semaphore *sem)
 146 {
 147         if (IS_ERR(____down_write(sem, "call_rwsem_down_write_failed_killable")))
 148                 return -EINTR;
 149
 150         return 0;
 151 }
 152
 153 /*
 154  * trylock for writing -- returns 1 if successful, 0 if contention
 155  */
 156 static inline bool __down_write_trylock(struct rw_semaphore *sem)
 157 {
 158         bool result;
 159         long tmp0, tmp1;
 160         asm volatile("# beginning __down_write_trylock\n\t"
 161                      "  mov          %[count],%[tmp0]\n\t"
 162                      "1:\n\t"
 163                      "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
 164                      /* was the active mask 0 before? */
 165                      "  jnz          2f\n\t"
 166                      "  mov          %[tmp0],%[tmp1]\n\t"
 167                      "  add          %[inc],%[tmp1]\n\t"
 168                      LOCK_PREFIX "  cmpxchg  %[tmp1],%[count]\n\t"
 169                      "  jnz          1b\n\t"
 170                      "2:\n\t"
 171                      CC_SET(e)
 172                      "# ending __down_write_trylock\n\t"
 173                      : [count] "+m" (sem->count), [tmp0] "=&a" (tmp0),
 174                        [tmp1] "=&r" (tmp1), CC_OUT(e) (result)
 175                      : [inc] "er" (RWSEM_ACTIVE_WRITE_BIAS)
 176                      : "memory");
 177         return result;
 178 }
 179
 180 /*
 181  * unlock after reading
 182  */
 183 static inline void __up_read(struct rw_semaphore *sem)
 184 {
 185         long tmp;
 186         asm volatile("# beginning __up_read\n\t"
 187                      LOCK_PREFIX "  xadd      %[tmp],(%[sem])\n\t"
 188                      /* subtracts 1, returns the old value */
 189                      "  jns        1f\n\t"
 190                      "  call call_rwsem_wake\n" /* expects old value in %edx */
 191                      "1:\n"
 192                      "# ending __up_read\n"
 193                      : "+m" (sem->count), [tmp] "=d" (tmp)
 194                      : [sem] "a" (sem), "[tmp]" (-RWSEM_ACTIVE_READ_BIAS)
 195                      : "memory", "cc");
 196 }
 197
 198 /*
 199  * unlock after writing
 200  */
 201 static inline void __up_write(struct rw_semaphore *sem)
 202 {
 203         long tmp;
 204         asm volatile("# beginning __up_write\n\t"
 205                      LOCK_PREFIX "  xadd      %[tmp],(%[sem])\n\t"
 206                      /* subtracts 0xffff0001, returns the old value */
 207                      "  jns        1f\n\t"
 208                      "  call call_rwsem_wake\n" /* expects old value in %edx */
 209                      "1:\n\t"
 210                      "# ending __up_write\n"
 211                      : "+m" (sem->count), [tmp] "=d" (tmp)
 212                      : [sem] "a" (sem), "[tmp]" (-RWSEM_ACTIVE_WRITE_BIAS)
 213                      : "memory", "cc");
 214 }
 215
 216 /*
 217  * downgrade write lock to read lock
 218  */
 219 static inline void __downgrade_write(struct rw_semaphore *sem)
 220 {
 221         asm volatile("# beginning __downgrade_write\n\t"
 222                      LOCK_PREFIX _ASM_ADD "%[inc],(%[sem])\n\t"
 223                      /*
 224                       * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
 225                       *     0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
 226                       */
 227                      "  jns       1f\n\t"
 228                      "  call call_rwsem_downgrade_wake\n"
 229                      "1:\n\t"
 230                      "# ending __downgrade_write\n"
 231                      : "+m" (sem->count)
 232                      : [sem] "a" (sem), [inc] "er" (-RWSEM_WAITING_BIAS)
 233                      : "memory", "cc");
 234 }
 235
 236 #endif /* __KERNEL__ */
 237 #endif /* _ASM_X86_RWSEM_H */