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>

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"

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__