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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_X86_MUTEX_32_H
  10 #define _ASM_X86_MUTEX_32_H
  11
  12 #include <asm/alternative.h>
  13
  14 /**
  15  *  __mutex_fastpath_lock - try to take the lock by moving the count
  16  *                          from 1 to a 0 value
  17  *  @count: pointer of type atomic_t
  18  *  @fn: function to call if the original value was not 1
  19  *
  20  * Change the count from 1 to a value lower than 1, and call <fn> if it
  21  * wasn't 1 originally. This function MUST leave the value lower than 1
  22  * even when the "1" assertion wasn't true.
  23  */
  24 #define __mutex_fastpath_lock(count, fail_fn)                   \
  25 do {                                                            \
  26         unsigned int dummy;                                     \
  27                                                                 \
  28         typecheck(atomic_t *, count);                           \
  29         typecheck_fn(void (*)(atomic_t *), fail_fn);            \
  30                                                                 \
  31         asm volatile(LOCK_PREFIX "   decl (%%eax)\n"            \
  32                      "   jns 1f \n"                             \
  33                      "   call " #fail_fn "\n"                   \
  34                      "1:\n"                                     \
  35                      : "=a" (dummy)                             \
  36                      : "a" (count)                              \
  37                      : "memory", "ecx", "edx");                 \
  38 } while (0)
  39
  40
  41 /**
  42  *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  43  *                                 from 1 to a 0 value
  44  *  @count: pointer of type atomic_t
  45  *  @fail_fn: function to call if the original value was not 1
  46  *
  47  * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
  48  * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  49  * or anything the slow path function returns
  50  */
  51 static inline int __mutex_fastpath_lock_retval(atomic_t *count,
  52                                                int (*fail_fn)(atomic_t *))
  53 {
  54         if (unlikely(atomic_dec_return(count) < 0))
  55                 return fail_fn(count);
  56         else
  57                 return 0;
  58 }
  59
  60 /**
  61  *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
  62  *  @count: pointer of type atomic_t
  63  *  @fail_fn: function to call if the original value was not 0
  64  *
  65  * try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>.
  66  * In the failure case, this function is allowed to either set the value
  67  * to 1, or to set it to a value lower than 1.
  68  *
  69  * If the implementation sets it to a value of lower than 1, the
  70  * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
  71  * to return 0 otherwise.
  72  */
  73 #define __mutex_fastpath_unlock(count, fail_fn)                 \
  74 do {                                                            \
  75         unsigned int dummy;                                     \
  76                                                                 \
  77         typecheck(atomic_t *, count);                           \
  78         typecheck_fn(void (*)(atomic_t *), fail_fn);            \
  79                                                                 \
  80         asm volatile(LOCK_PREFIX "   incl (%%eax)\n"            \
  81                      "   jg     1f\n"                           \
  82                      "   call " #fail_fn "\n"                   \
  83                      "1:\n"                                     \
  84                      : "=a" (dummy)                             \
  85                      : "a" (count)                              \
  86                      : "memory", "ecx", "edx");                 \
  87 } while (0)
  88
  89 #define __mutex_slowpath_needs_to_unlock()      1
  90
  91 /**
  92  * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  93  *
  94  *  @count: pointer of type atomic_t
  95  *  @fail_fn: fallback function
  96  *
  97  * Change the count from 1 to a value lower than 1, and return 0 (failure)
  98  * if it wasn't 1 originally, or return 1 (success) otherwise. This function
  99  * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
 100  * Additionally, if the value was < 0 originally, this function must not leave
 101  * it to 0 on failure.
 102  */
 103 static inline int __mutex_fastpath_trylock(atomic_t *count,
 104                                            int (*fail_fn)(atomic_t *))
 105 {
 106         /*
 107          * We have two variants here. The cmpxchg based one is the best one
 108          * because it never induce a false contention state.  It is included
 109          * here because architectures using the inc/dec algorithms over the
 110          * xchg ones are much more likely to support cmpxchg natively.
 111          *
 112          * If not we fall back to the spinlock based variant - that is
 113          * just as efficient (and simpler) as a 'destructive' probing of
 114          * the mutex state would be.
 115          */
 116 #ifdef __HAVE_ARCH_CMPXCHG
 117         if (likely(atomic_cmpxchg(count, 1, 0) == 1))
 118                 return 1;
 119         return 0;
 120 #else
 121         return fail_fn(count);
 122 #endif
 123 }
 124
 125 #endif /* _ASM_X86_MUTEX_32_H */