include/asm-x86_64/rwsem.h

   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 consequtive 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 dont 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
  51 /*
  52  * the semaphore definition
  53  */
  54 struct rw_semaphore {
  55         signed int              count;
  56 #define RWSEM_UNLOCKED_VALUE            0x00000000
  57 #define RWSEM_ACTIVE_BIAS               0x00000001
  58 #define RWSEM_ACTIVE_MASK               0x0000ffff
  59 #define RWSEM_WAITING_BIAS              (-0x00010000)
  60 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  61 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  62         spinlock_t              wait_lock;
  63         struct list_head        wait_list;
  64 #if RWSEM_DEBUG
  65         int                     debug;
  66 #endif
  67 };
  68
  69 /*
  70  * initialisation
  71  */
  72 #if RWSEM_DEBUG
  73 #define __RWSEM_DEBUG_INIT      , 0
  74 #else
  75 #define __RWSEM_DEBUG_INIT      /* */
  76 #endif
  77
  78 #define __RWSEM_INITIALIZER(name) \
  79 { RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
  80         __RWSEM_DEBUG_INIT }
  81
  82 #define DECLARE_RWSEM(name) \
  83         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  84
  85 static inline void init_rwsem(struct rw_semaphore *sem)
  86 {
  87         sem->count = RWSEM_UNLOCKED_VALUE;
  88         spin_lock_init(&sem->wait_lock);
  89         INIT_LIST_HEAD(&sem->wait_list);
  90 #if RWSEM_DEBUG
  91         sem->debug = 0;
  92 #endif
  93 }
  94
  95 /*
  96  * lock for reading
  97  */
  98 static inline void __down_read(struct rw_semaphore *sem)
  99 {
 100         __asm__ __volatile__(
 101                 "# beginning down_read\n\t"
 102 LOCK_PREFIX     "  incl      (%%rdi)\n\t" /* adds 0x00000001, returns the old value */
 103                 "  js        2f\n\t" /* jump if we weren't granted the lock */
 104                 "1:\n\t"
 105                 LOCK_SECTION_START("") \
 106                 "2:\n\t"
 107                 "  call      rwsem_down_read_failed_thunk\n\t"
 108                 "  jmp       1b\n"
 109                 LOCK_SECTION_END \
 110                 "# ending down_read\n\t"
 111                 : "+m"(sem->count)
 112                 : "D"(sem)
 113                 : "memory", "cc");
 114 }
 115
 116 /*
 117  * lock for writing
 118  */
 119 static inline void __down_write(struct rw_semaphore *sem)
 120 {
 121         int tmp;
 122
 123         tmp = RWSEM_ACTIVE_WRITE_BIAS;
 124         __asm__ __volatile__(
 125                 "# beginning down_write\n\t"
 126 LOCK_PREFIX     "  xaddl      %0,(%%rdi)\n\t" /* subtract 0x0000ffff, returns the old value */
 127                 "  testl     %0,%0\n\t" /* was the count 0 before? */
 128                 "  jnz       2f\n\t" /* jump if we weren't granted the lock */
 129                 "1:\n\t"
 130                 LOCK_SECTION_START("")
 131                 "2:\n\t"
 132                 "  call      rwsem_down_write_failed_thunk\n\t"
 133                 "  jmp       1b\n"
 134                 LOCK_SECTION_END
 135                 "# ending down_write"
 136                 : "=&r" (tmp)
 137                 : "0"(tmp), "D"(sem)
 138                 : "memory", "cc");
 139 }
 140
 141 /*
 142  * unlock after reading
 143  */
 144 static inline void __up_read(struct rw_semaphore *sem)
 145 {
 146         __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
 147         __asm__ __volatile__(
 148                 "# beginning __up_read\n\t"
 149 LOCK_PREFIX     "  xaddl      %%edx,(%%rdi)\n\t" /* subtracts 1, returns the old value */
 150                 "  js        2f\n\t" /* jump if the lock is being waited upon */
 151                 "1:\n\t"
 152                 LOCK_SECTION_START("")
 153                 "2:\n\t"
 154                 "  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
 155                 "  jnz       1b\n\t"
 156                 "  call      rwsem_wake_thunk\n\t"
 157                 "  jmp       1b\n"
 158                 LOCK_SECTION_END
 159                 "# ending __up_read\n"
 160                 : "+m"(sem->count), "+d"(tmp)
 161                 : "D"(sem)
 162                 : "memory", "cc");
 163 }
 164
 165 /*
 166  * unlock after writing
 167  */
 168 static inline void __up_write(struct rw_semaphore *sem)
 169 {
 170         __asm__ __volatile__(
 171                 "# beginning __up_write\n\t"
 172                 "  movl      %2,%%edx\n\t"
 173 LOCK_PREFIX     "  xaddl     %%edx,(%%rdi)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
 174                 "  jnz       2f\n\t" /* jump if the lock is being waited upon */
 175                 "1:\n\t"
 176                 LOCK_SECTION_START("")
 177                 "2:\n\t"
 178                 "  decw      %%dx\n\t" /* did the active count reduce to 0? */
 179                 "  jnz       1b\n\t" /* jump back if not */
 180                 "  call      rwsem_wake_thunk\n\t"
 181                 "  jmp       1b\n"
 182                 LOCK_SECTION_END
 183                 "# ending __up_write\n"
 184                 : "+m"(sem->count)
 185                 : "D"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS)
 186                 : "memory", "cc", "rdx");
 187 }
 188
 189 /*
 190  * implement atomic add functionality
 191  */
 192 static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 193 {
 194         __asm__ __volatile__(
 195 LOCK_PREFIX     "addl %1,%0"
 196                 :"=m"(sem->count)
 197                 :"ir"(delta), "m"(sem->count));
 198 }
 199
 200 /*
 201  * implement exchange and add functionality
 202  */
 203 static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 204 {
 205         int tmp = delta;
 206
 207         __asm__ __volatile__(
 208 LOCK_PREFIX     "xaddl %0,(%2)"
 209                 : "=r"(tmp), "=m"(sem->count)
 210                 : "r"(sem), "m"(sem->count), "0" (tmp)
 211                 : "memory");
 212
 213         return tmp+delta;
 214 }
 215
 216 #endif /* __KERNEL__ */
 217 #endif /* _X8664_RWSEM_H */