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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(LOCK_PREFIX "   decl (%%rdi)\n"            \
  27                      "   jns 1f         \n"                     \
  28                      "   call " #fail_fn "\n"                   \
  29                      "1:"                                       \
  30                      : "=D" (dummy)                             \
  31                      : "D" (v)                                  \
  32                      : "rax", "rsi", "rdx", "rcx",              \
  33                        "r8", "r9", "r10", "r11", "memory");     \
  34 } while (0)
  35
  36 /**
  37  *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  38  *                                 from 1 to a 0 value
  39  *  @count: pointer of type atomic_t
  40  *  @fail_fn: function to call if the original value was not 1
  41  *
  42  * Change the count from 1 to a value lower than 1, and call <fail_fn> if
  43  * it wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  44  * or anything the slow path function returns
  45  */
  46 static inline int __mutex_fastpath_lock_retval(atomic_t *count,
  47                                                int (*fail_fn)(atomic_t *))
  48 {
  49         if (unlikely(atomic_dec_return(count) < 0))
  50                 return fail_fn(count);
  51         else
  52                 return 0;
  53 }
  54
  55 /**
  56  * __mutex_fastpath_unlock - increment and call function if nonpositive
  57  * @v: pointer of type atomic_t
  58  * @fail_fn: function to call if the result is nonpositive
  59  *
  60  * Atomically increments @v and calls <fail_fn> if the result is nonpositive.
  61  */
  62 #define __mutex_fastpath_unlock(v, fail_fn)                     \
  63 do {                                                            \
  64         unsigned long dummy;                                    \
  65                                                                 \
  66         typecheck(atomic_t *, v);                               \
  67         typecheck_fn(void (*)(atomic_t *), fail_fn);            \
  68                                                                 \
  69         asm volatile(LOCK_PREFIX "   incl (%%rdi)\n"            \
  70                      "   jg 1f\n"                               \
  71                      "   call " #fail_fn "\n"                   \
  72                      "1:"                                       \
  73                      : "=D" (dummy)                             \
  74                      : "D" (v)                                  \
  75                      : "rax", "rsi", "rdx", "rcx",              \
  76                        "r8", "r9", "r10", "r11", "memory");     \
  77 } while (0)
  78
  79 #define __mutex_slowpath_needs_to_unlock()      1
  80
  81 /**
  82  * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  83  *
  84  *  @count: pointer of type atomic_t
  85  *  @fail_fn: fallback function
  86  *
  87  * Change the count from 1 to 0 and return 1 (success), or return 0 (failure)
  88  * if it wasn't 1 originally. [the fallback function is never used on
  89  * x86_64, because all x86_64 CPUs have a CMPXCHG instruction.]
  90  */
  91 static inline int __mutex_fastpath_trylock(atomic_t *count,
  92                                            int (*fail_fn)(atomic_t *))
  93 {
  94         if (likely(atomic_cmpxchg(count, 1, 0) == 1))
  95                 return 1;
  96         else
  97                 return 0;
  98 }
  99
 100 #endif