[PATCH v9 01/15] asm-generic: add barrier smp_cond_load_relaxed_timeout()

[Ankur Arora <ankur.a.arora@oracle.com>]

Add a timed variant of smp_cond_load_relaxed().

This is useful because arm64 supports polling on a conditional variable
by directly waiting on the cacheline instead of spin waiting for the
condition to change.

However, an implementation such as this has a problem that it can block
forever -- unless there's an explicit timeout or another out-of-band
mechanism which allows it to come out of the wait state periodically.

smp_cond_load_relaxed_timeout() supports these semantics by specifying
a time-check expression and an associated time-limit.

However, note that for the generic spin-wait implementation we want to
minimize the numbers of instructions executed in each iteration. So,
limit how often we evaluate the time-check expression by doing it once
every smp_cond_time_check_count.

The inner loop in poll_idle() has a substantially similar structure
and constraints as smp_cond_load_relaxed_timeout(), so define
smp_cond_time_check_count to the same value used in poll_idle().

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 include/asm-generic/barrier.h | 42 +++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..77726ef807e4 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,48 @@ do {									\
 })
 #endif
 
+#ifndef smp_cond_time_check_count
+/*
+ * Limit how often smp_cond_load_relaxed_timeout() evaluates time_expr_ns.
+ * This helps reduce the number of instructions executed while spin-waiting.
+ */
+#define smp_cond_time_check_count	200
+#endif
+
+/**
+ * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
+ * guarantees until a timeout expires.
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ * @time_expr_ns: evaluates to the current time
+ * @time_limit_ns: compared against time_expr_ns
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ *
+ * Due to C lacking lambda expressions we load the value of *ptr into a
+ * pre-named variable @VAL to be used in @cond.
+ */
+#ifndef smp_cond_load_relaxed_timeout
+#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr_ns,	\
+				      time_limit_ns) ({			\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	unsigned int __count = 0;					\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		cpu_relax();						\
+		if (__count++ < smp_cond_time_check_count)		\
+			continue;					\
+		if ((time_expr_ns) >= time_limit_ns)			\
+			break;						\
+		__count = 0;						\
+	}								\
+	(typeof(*ptr))VAL;						\
+})
+#endif
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
-- 
2.43.5