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   1 /*
   2  * Assembly implementation of the mutex fastpath, based on atomic
   3  * decrement/increment.
   4  *
   5  * started by Ingo Molnar:
   6  *
   7  *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
   8  */
   9 #ifndef _ASM_MUTEX_H
  10 #define _ASM_MUTEX_H
  11
  12 /**
  13  * __mutex_fastpath_lock - decrement and call function if negative
  14  * @v: pointer of type atomic_t
  15  * @fail_fn: function to call if the result is negative
  16  *
  17  * Atomically decrements @v and calls <fail_fn> if the result is negative.
  18  */
  19 #define __mutex_fastpath_lock(v, fail_fn)                               \
  20 do {                                                                    \
  21         unsigned long dummy;                                            \
  22                                                                         \
  23         typecheck(atomic_t *, v);                                       \
  24         typecheck_fn(void (*)(atomic_t *), fail_fn);                    \
  25                                                                         \
  26         __asm__ __volatile__(                                           \
  27                 LOCK_PREFIX "   decl (%%rdi)    \n"                     \
  28                         "   jns 1f              \n"                     \
  29                         "   call "#fail_fn"     \n"                     \
  30                         "1:"                                            \
  31                                                                         \
  32                 :"=D" (dummy)                                           \
  33                 : "D" (v)                                               \
  34                 : "rax", "rsi", "rdx", "rcx",                           \
  35                   "r8", "r9", "r10", "r11", "memory");                  \
  36 } while (0)
  37
  38 /**
  39  *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  40  *                                 from 1 to a 0 value
  41  *  @count: pointer of type atomic_t
  42  *  @fail_fn: function to call if the original value was not 1
  43  *
  44  * Change the count from 1 to a value lower than 1, and call <fail_fn> if
  45  * it wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  46  * or anything the slow path function returns
  47  */
  48 static inline int
  49 __mutex_fastpath_lock_retval(atomic_t *count,
  50                              int (*fail_fn)(atomic_t *))
  51 {
  52         if (unlikely(atomic_dec_return(count) < 0))
  53                 return fail_fn(count);
  54         else
  55                 return 0;
  56 }
  57
  58 /**
  59  * __mutex_fastpath_unlock - increment and call function if nonpositive
  60  * @v: pointer of type atomic_t
  61  * @fail_fn: function to call if the result is nonpositive
  62  *
  63  * Atomically increments @v and calls <fail_fn> if the result is nonpositive.
  64  */
  65 #define __mutex_fastpath_unlock(v, fail_fn)                             \
  66 do {                                                                    \
  67         unsigned long dummy;                                            \
  68                                                                         \
  69         typecheck(atomic_t *, v);                                       \
  70         typecheck_fn(void (*)(atomic_t *), fail_fn);                    \
  71                                                                         \
  72         __asm__ __volatile__(                                           \
  73                 LOCK_PREFIX "   incl (%%rdi)    \n"                     \
  74                         "   jg 1f               \n"                     \
  75                         "   call "#fail_fn"     \n"                     \
  76                         "1:                       "                     \
  77                                                                         \
  78                 :"=D" (dummy)                                           \
  79                 : "D" (v)                                               \
  80                 : "rax", "rsi", "rdx", "rcx",                           \
  81                   "r8", "r9", "r10", "r11", "memory");                  \
  82 } while (0)
  83
  84 #define __mutex_slowpath_needs_to_unlock()      1
  85
  86 /**
  87  * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  88  *
  89  *  @count: pointer of type atomic_t
  90  *  @fail_fn: fallback function
  91  *
  92  * Change the count from 1 to 0 and return 1 (success), or return 0 (failure)
  93  * if it wasn't 1 originally. [the fallback function is never used on
  94  * x86_64, because all x86_64 CPUs have a CMPXCHG instruction.]
  95  */
  96 static inline int
  97 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  98 {
  99         if (likely(atomic_cmpxchg(count, 1, 0) == 1))
 100                 return 1;
 101         else
 102                 return 0;
 103 }
 104
 105 #endif