spin loop primitives for busy waiting

[linux-2.6-microblaze.git] / include / linux / processor.h

diff --git a/include/linux/processor.h b/include/linux/processor.h
new file mode 100644 (file)
index 0000000..da0c5e5
--- /dev/null
+++ b/include/linux/processor.h
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+/* Misc low level processor primitives */
+#ifndef _LINUX_PROCESSOR_H
+#define _LINUX_PROCESSOR_H
+
+#include <asm/processor.h>
+
+/*
+ * spin_begin is used before beginning a busy-wait loop, and must be paired
+ * with spin_end when the loop is exited. spin_cpu_relax must be called
+ * within the loop.
+ *
+ * The loop body should be as small and fast as possible, on the order of
+ * tens of instructions/cycles as a guide. It should and avoid calling
+ * cpu_relax, or any "spin" or sleep type of primitive including nested uses
+ * of these primitives. It should not lock or take any other resource.
+ * Violations of these guidelies will not cause a bug, but may cause sub
+ * optimal performance.
+ *
+ * These loops are optimized to be used where wait times are expected to be
+ * less than the cost of a context switch (and associated overhead).
+ *
+ * Detection of resource owner and decision to spin or sleep or guest-yield
+ * (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be
+ * tested within the loop body.
+ */
+#ifndef spin_begin
+#define spin_begin()
+#endif
+
+#ifndef spin_cpu_relax
+#define spin_cpu_relax() cpu_relax()
+#endif
+
+/*
+ * spin_cpu_yield may be called to yield (undirected) to the hypervisor if
+ * necessary. This should be used if the wait is expected to take longer
+ * than context switch overhead, but we can't sleep or do a directed yield.
+ */
+#ifndef spin_cpu_yield
+#define spin_cpu_yield() cpu_relax_yield()
+#endif
+
+#ifndef spin_end
+#define spin_end()
+#endif
+
+/*
+ * spin_until_cond can be used to wait for a condition to become true. It
+ * may be expected that the first iteration will true in the common case
+ * (no spinning), so that callers should not require a first "likely" test
+ * for the uncontended case before using this primitive.
+ *
+ * Usage and implementation guidelines are the same as for the spin_begin
+ * primitives, above.
+ */
+#ifndef spin_until_cond
+#define spin_until_cond(cond)                                  \
+do {                                                           \
+       if (unlikely(!(cond))) {                                \
+               spin_begin();                                   \
+               do {                                            \
+                       spin_cpu_relax();                       \
+               } while (!(cond));                              \
+               spin_end();                                     \
+       }                                                       \
+} while (0)
+
+#endif
+
+#endif /* _LINUX_PROCESSOR_H */