From: Dmitri Shubin <dmitri.shubin@orc-group.com>
Date: Fri, 27 Mar 2015 19:34:11 +0000 (-0400)
Subject: Add tracepoint_enabled() macro
X-Git-Tag: v2.7.0-rc1~50
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=8da6d0c8216f13df8f714459b7d855243296d83c;p=lttng-ust.git

Add tracepoint_enabled() macro

Allows application to check if given probe is enabled.
It's useful when data it passes to tracepoint is expensive to compute.

Signed-off-by: Dmitri Shubin <dmitri.shubin@orc-group.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/doc/man/lttng-ust.3 b/doc/man/lttng-ust.3
index 6f8763b9..7dae8b53 100644
--- a/doc/man/lttng-ust.3
+++ b/doc/man/lttng-ust.3
@@ -290,6 +290,33 @@ Even though LTTng-UST supports tracepoint() call site duplicates having
 the same provider and event name, it is recommended to use a
 provider event name pair only once within the source code to help
 map events back to their call sites when analyzing the trace.
+
+Sometimes arguments to the probe are expensive to compute (e.g.
+take call stack). To avoid the computation when the tracepoint is
+disabled one can use more 'low level' tracepoint_enabled() and
+do_tracepoint() macros as following:
+
+	if (tracepoint_enabled(ust_tests_hello, tptest)) {
+		/* prepare arguments */
+		do_tracepoint(ust_tests_hello, tptest, i, netint, values,
+			text, strlen(text), dbl, flt);
+	}
+
+Here do_tracepoint() doesn't contain check if the tracepoint is enabled.
+Using tracepoint() in such scenario is dangerous since it also contains
+enabled check and thus race condition is possible in the following code
+if the tracepoint has been enabled after check in tracepoint_enabled()
+but before tracepoint():
+
+	if (tracepoint_enabled(provider, name)) { /* tracepoint is disabled */
+		prepare(args);
+	}
+	/* tracepoint is enabled by 'lttng' tool */
+	tracepoint(provider, name, args); /* args wasn't prepared properly */
+
+Note also that neither tracepoint_enabled() nor do_tracepoint() have
+STAP_PROBEV() call so if you need it you should emit this call yourself.
+
 .fi
 
 .SH "BUILDING/LINKING THE TRACEPOINT PROVIDER"
diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index 63759a20..1734c1b7 100644
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -44,11 +44,17 @@
 extern "C" {
 #endif
 
+#define tracepoint_enabled(provider, name) \
+	caa_unlikely(__tracepoint_##provider##___##name.state)
+
+#define do_tracepoint(provider, name, ...) \
+	__tracepoint_cb_##provider##___##name(__VA_ARGS__)
+
 #define tracepoint(provider, name, ...)					    \
 	do {								    \
 		STAP_PROBEV(provider, name, ## __VA_ARGS__);		    \
-		if (caa_unlikely(__tracepoint_##provider##___##name.state)) \
-			__tracepoint_cb_##provider##___##name(__VA_ARGS__); \
+		if (tracepoint_enabled(provider, name)) 		    \
+			do_tracepoint(provider, name, __VA_ARGS__);	    \
 	} while (0)
 
 #define TP_ARGS(...)       __VA_ARGS__