include/linux/processor.h

0001 /* SPDX-License-Identifier: GPL-2.0 */
0002 /* Misc low level processor primitives */
0003 #ifndef _LINUX_PROCESSOR_H
0004 #define _LINUX_PROCESSOR_H
0005
0006 #include <asm/processor.h>
0007
0008 /*
0009  * spin_begin is used before beginning a busy-wait loop, and must be paired
0010  * with spin_end when the loop is exited. spin_cpu_relax must be called
0011  * within the loop.
0012  *
0013  * The loop body should be as small and fast as possible, on the order of
0014  * tens of instructions/cycles as a guide. It should and avoid calling
0015  * cpu_relax, or any "spin" or sleep type of primitive including nested uses
0016  * of these primitives. It should not lock or take any other resource.
0017  * Violations of these guidelies will not cause a bug, but may cause sub
0018  * optimal performance.
0019  *
0020  * These loops are optimized to be used where wait times are expected to be
0021  * less than the cost of a context switch (and associated overhead).
0022  *
0023  * Detection of resource owner and decision to spin or sleep or guest-yield
0024  * (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be
0025  * tested within the loop body.
0026  */
0027 #ifndef spin_begin
0028 #define spin_begin()
0029 #endif
0030
0031 #ifndef spin_cpu_relax
0032 #define spin_cpu_relax() cpu_relax()
0033 #endif
0034
0035 #ifndef spin_end
0036 #define spin_end()
0037 #endif
0038
0039 /*
0040  * spin_until_cond can be used to wait for a condition to become true. It
0041  * may be expected that the first iteration will true in the common case
0042  * (no spinning), so that callers should not require a first "likely" test
0043  * for the uncontended case before using this primitive.
0044  *
0045  * Usage and implementation guidelines are the same as for the spin_begin
0046  * primitives, above.
0047  */
0048 #ifndef spin_until_cond
0049 #define spin_until_cond(cond)                   \
0050 do {                                \
0051     if (unlikely(!(cond))) {                \
0052         spin_begin();                   \
0053         do {                        \
0054             spin_cpu_relax();           \
0055         } while (!(cond));              \
0056         spin_end();                 \
0057     }                           \
0058 } while (0)
0059
0060 #endif
0061
0062 #endif /* _LINUX_PROCESSOR_H */