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