include/linux/processor.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 /* Misc low level processor primitives */
   3 #ifndef _LINUX_PROCESSOR_H
   4 #define _LINUX_PROCESSOR_H
   5
   6 #include <asm/processor.h>
   7
   8 /*
   9  * spin_begin is used before beginning a busy-wait loop, and must be paired
  10  * with spin_end when the loop is exited. spin_cpu_relax must be called
  11  * within the loop.
  12  *
  13  * The loop body should be as small and fast as possible, on the order of
  14  * tens of instructions/cycles as a guide. It should and avoid calling
  15  * cpu_relax, or any "spin" or sleep type of primitive including nested uses
  16  * of these primitives. It should not lock or take any other resource.
  17  * Violations of these guidelies will not cause a bug, but may cause sub
  18  * optimal performance.
  19  *
  20  * These loops are optimized to be used where wait times are expected to be
  21  * less than the cost of a context switch (and associated overhead).
  22  *
  23  * Detection of resource owner and decision to spin or sleep or guest-yield
  24  * (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be
  25  * tested within the loop body.
  26  */
  27 #ifndef spin_begin
  28 #define spin_begin()
  29 #endif
  30
  31 #ifndef spin_cpu_relax
  32 #define spin_cpu_relax() cpu_relax()
  33 #endif
  34
  35 #ifndef spin_end
  36 #define spin_end()
  37 #endif
  38
  39 /*
  40  * spin_until_cond can be used to wait for a condition to become true. It
  41  * may be expected that the first iteration will true in the common case
  42  * (no spinning), so that callers should not require a first "likely" test
  43  * for the uncontended case before using this primitive.
  44  *
  45  * Usage and implementation guidelines are the same as for the spin_begin
  46  * primitives, above.
  47  */
  48 #ifndef spin_until_cond
  49 #define spin_until_cond(cond)                                   \
  50 do {                                                            \
  51         if (unlikely(!(cond))) {                                \
  52                 spin_begin();                                   \
  53                 do {                                            \
  54                         spin_cpu_relax();                       \
  55                 } while (!(cond));                              \
  56                 spin_end();                                     \
  57         }                                                       \
  58 } while (0)
  59
  60 #endif
  61
  62 #endif /* _LINUX_PROCESSOR_H */