include/linux/processor.h

   1 /* Misc low level processor primitives */
   2 #ifndef _LINUX_PROCESSOR_H
   3 #define _LINUX_PROCESSOR_H
   4
   5 #include <asm/processor.h>
   6
   7 /*
   8  * spin_begin is used before beginning a busy-wait loop, and must be paired
   9  * with spin_end when the loop is exited. spin_cpu_relax must be called
  10  * within the loop.
  11  *
  12  * The loop body should be as small and fast as possible, on the order of
  13  * tens of instructions/cycles as a guide. It should and avoid calling
  14  * cpu_relax, or any "spin" or sleep type of primitive including nested uses
  15  * of these primitives. It should not lock or take any other resource.
  16  * Violations of these guidelies will not cause a bug, but may cause sub
  17  * optimal performance.
  18  *
  19  * These loops are optimized to be used where wait times are expected to be
  20  * less than the cost of a context switch (and associated overhead).
  21  *
  22  * Detection of resource owner and decision to spin or sleep or guest-yield
  23  * (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be
  24  * tested within the loop body.
  25  */
  26 #ifndef spin_begin
  27 #define spin_begin()
  28 #endif
  29
  30 #ifndef spin_cpu_relax
  31 #define spin_cpu_relax() cpu_relax()
  32 #endif
  33
  34 /*
  35  * spin_cpu_yield may be called to yield (undirected) to the hypervisor if
  36  * necessary. This should be used if the wait is expected to take longer
  37  * than context switch overhead, but we can't sleep or do a directed yield.
  38  */
  39 #ifndef spin_cpu_yield
  40 #define spin_cpu_yield() cpu_relax_yield()
  41 #endif
  42
  43 #ifndef spin_end
  44 #define spin_end()
  45 #endif
  46
  47 /*
  48  * spin_until_cond can be used to wait for a condition to become true. It
  49  * may be expected that the first iteration will true in the common case
  50  * (no spinning), so that callers should not require a first "likely" test
  51  * for the uncontended case before using this primitive.
  52  *
  53  * Usage and implementation guidelines are the same as for the spin_begin
  54  * primitives, above.
  55  */
  56 #ifndef spin_until_cond
  57 #define spin_until_cond(cond)                                   \
  58 do {                                                            \
  59         if (unlikely(!(cond))) {                                \
  60                 spin_begin();                                   \
  61                 do {                                            \
  62                         spin_cpu_relax();                       \
  63                 } while (!(cond));                              \
  64                 spin_end();                                     \
  65         }                                                       \
  66 } while (0)
  67
  68 #endif
  69
  70 #endif /* _LINUX_PROCESSOR_H */