From 6657edecc8dd4f294c42339645151006f60b3811 Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 30 Nov 2020 11:11:39 -0500
Subject: [PATCH] Introduce lttng smp_store_release smp_load_acquire wrappers

Kernels prior to 3.14 do not implement smp_store_release nor
smp_load_acquire. Implement our own wrappers with smp_mb instead for
those older kernels.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 include/wrapper/barrier.h               | 47 +++++++++++++++++++++++++
 src/lttng-abi.c                         |  3 +-
 src/lttng-event-notifier-notification.c |  5 +--
 3 files changed, 52 insertions(+), 3 deletions(-)
 create mode 100644 include/wrapper/barrier.h

diff --git a/include/wrapper/barrier.h b/include/wrapper/barrier.h
new file mode 100644
index 00000000..c2f0545b
--- /dev/null
+++ b/include/wrapper/barrier.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0-only
+ *
+ * wrapper/barrier.h
+ *
+ * wrapper around asm/barrier.h.
+ *
+ * Copyright (C) 2020 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ */
+
+#ifndef _LTTNG_WRAPPER_BARRIER_H
+#define _LTTNG_WRAPPER_BARRIER_H
+
+#include <linux/version.h>
+#include <asm/barrier.h>
+
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(3,14,0)
+
+#define lttng_smp_store_release(x, v) smp_store_release(x, v)
+#define lttng_smp_load_acquire(x) smp_load_acquire(x)
+
+#else
+
+/*
+ * Acquire-release semantics act as a one-way permeable barrier when
+ * pairing a store with a release. Use a full memory barrier to emulate
+ * the acquire-release semantic with a stronger barrier on older
+ * kernels.
+ */
+
+#define lttng_smp_store_release(x, v)		\
+	do {					\
+		smp_mb();			\
+		ACCESS_ONCE(*(x)) = (v);	\
+	} while (0)
+
+#define lttng_smp_load_acquire(x)		\
+	({					\
+		__typeof__(*(x)) ___ret;	\
+						\
+		___ret = ACCESS_ONCE(*(x));	\
+		smp_mb();			\
+		___ret;				\
+	})
+
+#endif
+
+#endif /* _LTTNG_WRAPPER_BARRIER_H */
diff --git a/src/lttng-abi.c b/src/lttng-abi.c
index 83853436..7096daf7 100644
--- a/src/lttng-abi.c
+++ b/src/lttng-abi.c
@@ -37,6 +37,7 @@
 #include <wrapper/poll.h>
 #include <wrapper/file.h>
 #include <wrapper/kref.h>
+#include <wrapper/barrier.h>
 #include <lttng/string-utils.h>
 #include <lttng/abi.h>
 #include <lttng/abi-old.h>
@@ -2150,7 +2151,7 @@ long lttng_abi_event_notifier_group_create_error_counter(
 	 * in record_error. Ensures the counter is created and the
 	 * error_counter_len is set before they are used.
 	 */
-	smp_store_release(&event_notifier_group->error_counter, counter);
+	lttng_smp_store_release(&event_notifier_group->error_counter, counter);
 
 	counter->file = counter_file;
 	counter->owner = event_notifier_group->file;
diff --git a/src/lttng-event-notifier-notification.c b/src/lttng-event-notifier-notification.c
index 77e72842..52b5593a 100644
--- a/src/lttng-event-notifier-notification.c
+++ b/src/lttng-event-notifier-notification.c
@@ -11,6 +11,7 @@
 #include <lttng/events.h>
 #include <lttng/msgpack.h>
 #include <lttng/event-notifier-notification.h>
+#include <wrapper/barrier.h>
 
 /*
  * The capture buffer size needs to be below 1024 bytes to avoid the
@@ -357,11 +358,11 @@ void record_error(struct lttng_event_notifier *event_notifier)
 	int ret;
 
 	/*
-	 * smp_load_acquire paired with smp_store_release orders
+	 * lttng_smp_load_acquire paired with lttng_smp_store_release orders
 	 * creation of the error counter and setting error_counter_len
 	 * before the error_counter is used.
 	 */
-	error_counter = smp_load_acquire(&event_notifier_group->error_counter);
+	error_counter = lttng_smp_load_acquire(&event_notifier_group->error_counter);
 	/* This group may not have an error counter attached to it. */
 	if (!error_counter)
 		return;
-- 
2.34.1