[PATCH -v3 2/8] locking: Introduce cmpwait()

[Peter Zijlstra]

Provide the cmpwait() primitive, which will 'spin' wait for a variable
to change and use it to implement smp_cond_load_acquire().

This primitive can be implemented with hardware assist on some
platforms (ARM64, x86).

Suggested-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/compiler.h |   19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -305,6 +305,23 @@ static __always_inline void __write_once
 })
 
 /**
+ * cmpwait - compare and wait for a variable to change
+ * @ptr: pointer to the variable to wait on
+ * @val: the value it should change from
+ *
+ * A simple constuct that waits for a variable to change from a known
+ * value; some architectures can do this in hardware.
+ */
+#ifndef cmpwait
+#define cmpwait(ptr, val) do {					\
+	typeof (ptr) __ptr = (ptr);				\
+	typeof (val) __val = (val);				\
+	while (READ_ONCE(*__ptr) == __val)			\
+		cpu_relax();					\
+} while (0)
+#endif
+
+/**
  * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
  * @ptr: pointer to the variable to wait on
  * @cond: boolean expression to wait for
@@ -327,7 +344,7 @@ static __always_inline void __write_once
 		VAL = READ_ONCE(*__PTR);			\
 		if (cond_expr)					\
 			break;					\
-		cpu_relax();					\
+		cmpwait(__PTR, VAL);				\
 	}							\
 	smp_rmb(); /* ctrl + rmb := acquire */			\
 	VAL;							\