From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 9 Dec 2020 14:30:23 +0000 (-0500)
Subject: Fix: notifier: use store-release/load-acquire for error counter
X-Git-Tag: v2.13.0-rc1~413
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=33f43caa08bc332900bc8671a156912a09e88aa2;p=lttng-ust.git

Fix: notifier: use store-release/load-acquire for error counter

The "record_error" operation is executed concurrently with setting the
error counter in the notifier group without locking, so we need to
explicitly provide existance guarantees.

The only visible transition is from NULL -> !NULL, because the only
situation reverting the error counter back to NULL is on destruction of
the notification group, after an RCU synchronisation guarantees that no
record_error can observe this pointer anymore.

Use the stronger "cmm_smp_mb()" to provide store-release/load-acquire
semantics because the explicit store-release and load-acquire are not
implemented in the CMM model yet.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I3bbeecc00ff33c5d8ad1e3fd41c8a0dc866f1943
---

diff --git a/liblttng-ust/event-notifier-notification.c b/liblttng-ust/event-notifier-notification.c
index 5f7d9d58..ed2c8d28 100644
--- a/liblttng-ust/event-notifier-notification.c
+++ b/liblttng-ust/event-notifier-notification.c
@@ -284,17 +284,26 @@ static void record_error(struct lttng_event_notifier *event_notifier)
 {
 	struct lttng_event_notifier_group *event_notifier_group =
 			event_notifier->group;
+	struct lttng_counter *error_counter;
 	size_t dimension_index[1];
 	int ret;
 
+	error_counter = CMM_LOAD_SHARED(event_notifier_group->error_counter);
+	/*
+	 * load-acquire paired with store-release orders creation of the
+	 * error counter and setting error_counter_len before the
+	 * error_counter is used.
+	 * Currently a full memory barrier is used, which could be
+	 * turned into acquire-release barriers.
+	 */
+	cmm_smp_mb();
 	/* This group may not have an error counter attached to it. */
-	if (!event_notifier_group->error_counter)
+	if (!error_counter)
 		return;
 
 	dimension_index[0] = event_notifier->error_counter_index;
 	ret = event_notifier_group->error_counter->ops->counter_add(
-			event_notifier_group->error_counter->counter,
-			dimension_index, 1);
+			error_counter->counter, dimension_index, 1);
 	if (ret)
 		WARN_ON_ONCE(1);
 }
diff --git a/liblttng-ust/lttng-ust-abi.c b/liblttng-ust/lttng-ust-abi.c
index a024b616..aad64a6d 100644
--- a/liblttng-ust/lttng-ust-abi.c
+++ b/liblttng-ust/lttng-ust-abi.c
@@ -846,8 +846,16 @@ int lttng_ust_event_notifier_group_create_error_counter(int event_notifier_group
 		goto create_error;
 	}
 
-	event_notifier_group->error_counter = counter;
 	event_notifier_group->error_counter_len = counter_len;
+	/*
+	 * store-release to publish error counter matches load-acquire
+	 * in record_error. Ensures the counter is created and the
+	 * error_counter_len is set before they are used.
+	 * Currently a full memory barrier is used, which could be
+	 * turned into acquire-release barriers.
+	 */
+	cmm_smp_mb();
+	CMM_STORE_SHARED(event_notifier_group->error_counter, counter);
 
 	counter->objd = counter_objd;
 	counter->event_notifier_group = event_notifier_group;	/* owner */