LXR linux/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)                                   \
  50do {                                                            \
  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 */
  63