From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 22 Jun 2015 14:30:02 +0000 (-0400)
Subject: Fix: perform volatile load of tracepoint state
X-Git-Tag: v2.6.3~15
X-Git-Url: http://git.liburcu.org/?p=lttng-ust.git;a=commitdiff_plain;h=05523fbf3fa3af6247a048c32d219b07bd65fd1a

Fix: perform volatile load of tracepoint state

Because tracepoint state is updated by lttng-ust threads, and read by
application threads, we need to perform a volatile load of that state.

Not having the volatile load can cause the compiler to optimize away the
reload of that state, keeping a local copy instead. For instance, a main
program consisting of a loop could keep the tracepoint state on its
stack or in registers without ever reloading it from memory.

Perform a volatile load (CMM_LOAD_SHARED()) of the tracepoint state.
Add the matching volatile store (CMM_STORE_SHARED()) in tracepoint.c for
the state update.

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

diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index 63759a20..39539911 100644
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -28,6 +28,7 @@
 #include <lttng/tracepoint-types.h>
 #include <lttng/tracepoint-rcu.h>
 #include <urcu/compiler.h>
+#include <urcu/system.h>
 #include <dlfcn.h>	/* for dlopen */
 #include <string.h>	/* for memset */
 #include <lttng/ust-config.h>	/* for sdt */
@@ -47,7 +48,7 @@ extern "C" {
 #define tracepoint(provider, name, ...)					    \
 	do {								    \
 		STAP_PROBEV(provider, name, ## __VA_ARGS__);		    \
-		if (caa_unlikely(__tracepoint_##provider##___##name.state)) \
+		if (caa_unlikely(CMM_LOAD_SHARED(__tracepoint_##provider##___##name.state))) \
 			__tracepoint_cb_##provider##___##name(__VA_ARGS__); \
 	} while (0)
 
diff --git a/liblttng-ust/tracepoint.c b/liblttng-ust/tracepoint.c
index a4a79ba5..5fd1126a 100644
--- a/liblttng-ust/tracepoint.c
+++ b/liblttng-ust/tracepoint.c
@@ -30,6 +30,7 @@
 #include <urcu/hlist.h>
 #include <urcu/uatomic.h>
 #include <urcu/compiler.h>
+#include <urcu/system.h>
 
 #include <lttng/tracepoint.h>
 #include <lttng/ust-abi.h>	/* for LTTNG_UST_SYM_NAME_LEN */
@@ -347,7 +348,7 @@ static void set_tracepoint(struct tracepoint_entry **entry,
 	 * is used.
 	 */
 	rcu_assign_pointer(elem->probes, (*entry)->probes);
-	elem->state = active;
+	CMM_STORE_SHARED(elem->state, active);
 }
 
 /*
@@ -358,7 +359,7 @@ static void set_tracepoint(struct tracepoint_entry **entry,
  */
 static void disable_tracepoint(struct lttng_ust_tracepoint *elem)
 {
-	elem->state = 0;
+	CMM_STORE_SHARED(elem->state, 0);
 	rcu_assign_pointer(elem->probes, NULL);
 }