[PATCH v7 1/7] asm-generic: barrier: Add smp_cond_load_relaxed_timeout()

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

Add smp_cond_load_relaxed_timeout(), which extends
smp_cond_load_relaxed() to allow waiting for a duration.

The waiting loop uses cpu_poll_relax() to wait on the condition
variable with a periodic evaluation of a time-check.

cpu_poll_relax() unless overridden by the arch code, amounts to
a cpu_relax().

The number of times we spin is defined by SMP_TIMEOUT_POLL_COUNT
(chosen to be 200 by default) which, assuming each cpu_poll_relax()
iteration takes around 20-30 cycles (measured on a variety of x86
platforms), for a total of ~4000-6000 cycles.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Haris Okanovic <harisokn@amazon.com>
Tested-by: Haris Okanovic <harisokn@amazon.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 include/asm-generic/barrier.h | 41 +++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..0063b46ec065 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,47 @@ do {									\
 })
 #endif
 
+#ifndef SMP_TIMEOUT_POLL_COUNT
+#define SMP_TIMEOUT_POLL_COUNT		200
+#endif
+
+#ifndef cpu_poll_relax
+#define cpu_poll_relax(ptr, val)	cpu_relax()
+#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_check_expr: expression to decide when to bail out
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ */
+#ifndef smp_cond_load_relaxed_timeout
+#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_check_expr)	\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT;			\
+									\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		cpu_poll_relax(__PTR, VAL);				\
+		if (++__n < __spin)					\
+			continue;					\
+		if (time_check_expr) {					\
+			VAL = READ_ONCE(*__PTR);			\
+			break;						\
+		}							\
+		__n = 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