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   1 /* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for x86_64+
   2  *
   3  * Written by David Howells (dhowells@redhat.com).
   4  * Ported by Andi Kleen <ak@suse.de> to x86-64.
   5  *
   6  * Derived from asm-i386/semaphore.h and asm-i386/rwsem.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 _X8664_RWSEM_H
  34 #define _X8664_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
  42 #include <linux/list.h>
  43 #include <linux/spinlock.h>
  44
  45 struct rwsem_waiter;
  46
  47 extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
  48 extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
  49 extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
  50 extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
  51
  52 /*
  53  * the semaphore definition
  54  */
  55 struct rw_semaphore {
  56         signed int              count;
  57 #define RWSEM_UNLOCKED_VALUE            0x00000000
  58 #define RWSEM_ACTIVE_BIAS               0x00000001
  59 #define RWSEM_ACTIVE_MASK               0x0000ffff
  60 #define RWSEM_WAITING_BIAS              (-0x00010000)
  61 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  62 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  63         spinlock_t              wait_lock;
  64         struct list_head        wait_list;
  65 #if RWSEM_DEBUG
  66         int                     debug;
  67 #endif
  68 };
  69
  70 /*
  71  * initialisation
  72  */
  73 #if RWSEM_DEBUG
  74 #define __RWSEM_DEBUG_INIT      , 0
  75 #else
  76 #define __RWSEM_DEBUG_INIT      /* */
  77 #endif
  78
  79 #define __RWSEM_INITIALIZER(name) \
  80 { RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
  81         __RWSEM_DEBUG_INIT }
  82
  83 #define DECLARE_RWSEM(name) \
  84         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  85
  86 static inline void init_rwsem(struct rw_semaphore *sem)
  87 {
  88         sem->count = RWSEM_UNLOCKED_VALUE;
  89         spin_lock_init(&sem->wait_lock);
  90         INIT_LIST_HEAD(&sem->wait_list);
  91 #if RWSEM_DEBUG
  92         sem->debug = 0;
  93 #endif
  94 }
  95
  96 /*
  97  * lock for reading
  98  */
  99 static inline void __down_read(struct rw_semaphore *sem)
 100 {
 101         __asm__ __volatile__(
 102                 "# beginning down_read\n\t"
 103 LOCK_PREFIX     "  incl      (%%rdi)\n\t" /* adds 0x00000001, returns the old value */
 104                 "  js        2f\n\t" /* jump if we weren't granted the lock */
 105                 "1:\n\t"
 106                 LOCK_SECTION_START("") \
 107                 "2:\n\t"
 108                 "  call      rwsem_down_read_failed_thunk\n\t"
 109                 "  jmp       1b\n"
 110                 LOCK_SECTION_END \
 111                 "# ending down_read\n\t"
 112                 : "+m"(sem->count)
 113                 : "D"(sem)
 114                 : "memory", "cc");
 115 }
 116
 117
 118 /*
 119  * trylock for reading -- returns 1 if successful, 0 if contention
 120  */
 121 static inline int __down_read_trylock(struct rw_semaphore *sem)
 122 {
 123         __s32 result, tmp;
 124         __asm__ __volatile__(
 125                 "# 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 /*
 143  * lock for writing
 144  */
 145 static inline void __down_write(struct rw_semaphore *sem)
 146 {
 147         int tmp;
 148
 149         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 150         __asm__ __volatile__(
 151                 "# beginning down_write\n\t"
 152 LOCK_PREFIX     "  xaddl      %0,(%%rdi)\n\t" /* subtract 0x0000ffff, returns the old value */
 153                 "  testl     %0,%0\n\t" /* was the count 0 before? */
 154                 "  jnz       2f\n\t" /* jump if we weren't granted the lock */
 155                 "1:\n\t"
 156                 LOCK_SECTION_START("")
 157                 "2:\n\t"
 158                 "  call      rwsem_down_write_failed_thunk\n\t"
 159                 "  jmp       1b\n"
 160                 LOCK_SECTION_END
 161                 "# ending down_write"
 162                 : "=&r" (tmp)
 163                 : "0"(tmp), "D"(sem)
 164                 : "memory", "cc");
 165 }
 166
 167 /*
 168  * trylock for writing -- returns 1 if successful, 0 if contention
 169  */
 170 static inline int __down_write_trylock(struct rw_semaphore *sem)
 171 {
 172         signed long ret = cmpxchg(&sem->count,
 173                                   RWSEM_UNLOCKED_VALUE,
 174                                   RWSEM_ACTIVE_WRITE_BIAS);
 175         if (ret == RWSEM_UNLOCKED_VALUE)
 176                 return 1;
 177         return 0;
 178 }
 179
 180 /*
 181  * unlock after reading
 182  */
 183 static inline void __up_read(struct rw_semaphore *sem)
 184 {
 185         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 186         __asm__ __volatile__(
 187                 "# beginning __up_read\n\t"
 188 LOCK_PREFIX     "  xaddl      %[tmp],(%%rdi)\n\t" /* subtracts 1, returns the old value */
 189                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 190                 "1:\n\t"
 191                 LOCK_SECTION_START("")
 192                 "2:\n\t"
 193                 "  decw      %w[tmp]\n\t" /* do nothing if still outstanding active readers */
 194                 "  jnz       1b\n\t"
 195                 "  call      rwsem_wake_thunk\n\t"
 196                 "  jmp       1b\n"
 197                 LOCK_SECTION_END
 198                 "# ending __up_read\n"
 199                 : "+m"(sem->count), [tmp] "+r" (tmp)
 200                 : "D"(sem)
 201                 : "memory", "cc");
 202 }
 203
 204 /*
 205  * unlock after writing
 206  */
 207 static inline void __up_write(struct rw_semaphore *sem)
 208 {
 209         unsigned tmp;
 210         __asm__ __volatile__(
 211                 "# beginning __up_write\n\t"
 212                 "  movl     %[bias],%[tmp]\n\t"
 213 LOCK_PREFIX     "  xaddl     %[tmp],(%%rdi)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
 214                 "  jnz       2f\n\t" /* jump if the lock is being waited upon */
 215                 "1:\n\t"
 216                 LOCK_SECTION_START("")
 217                 "2:\n\t"
 218                 "  decw      %w[tmp]\n\t" /* did the active count reduce to 0? */
 219                 "  jnz       1b\n\t" /* jump back if not */
 220                 "  call      rwsem_wake_thunk\n\t"
 221                 "  jmp       1b\n"
 222                 LOCK_SECTION_END
 223                 "# ending __up_write\n"
 224                 : "+m"(sem->count), [tmp] "=r" (tmp)
 225                 : "D"(sem), [bias] "i"(-RWSEM_ACTIVE_WRITE_BIAS)
 226                 : "memory", "cc");
 227 }
 228
 229 /*
 230  * downgrade write lock to read lock
 231  */
 232 static inline void __downgrade_write(struct rw_semaphore *sem)
 233 {
 234         __asm__ __volatile__(
 235                 "# beginning __downgrade_write\n\t"
 236 LOCK_PREFIX     "  addl      %[bias],(%%rdi)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
 237                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 238                 "1:\n\t"
 239                 LOCK_SECTION_START("")
 240                 "2:\n\t"
 241                 "  call      rwsem_downgrade_thunk\n"
 242                 "  jmp       1b\n"
 243                 LOCK_SECTION_END
 244                 "# ending __downgrade_write\n"
 245                 : "=m"(sem->count)
 246                 : "D"(sem), [bias] "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
 247                 : "memory", "cc");
 248 }
 249
 250 /*
 251  * implement atomic add functionality
 252  */
 253 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 254 {
 255         __asm__ __volatile__(
 256 LOCK_PREFIX     "addl %1,%0"
 257                 :"=m"(sem->count)
 258                 :"ir"(delta), "m"(sem->count));
 259 }
 260
 261 /*
 262  * implement exchange and add functionality
 263  */
 264 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 265 {
 266         int tmp = delta;
 267
 268         __asm__ __volatile__(
 269 LOCK_PREFIX     "xaddl %0,(%2)"
 270                 : "=r"(tmp), "=m"(sem->count)
 271                 : "r"(sem), "m"(sem->count), "0" (tmp)
 272                 : "memory");
 273
 274         return tmp+delta;
 275 }
 276
 277 #endif /* __KERNEL__ */
 278 #endif /* _X8664_RWSEM_H */