[PATCH lttng-ust] Add ctor/dtor priorities for tracepoints/events

[Olivier Dion via lttng-dev <lttng-dev@lists.lttng.org>]

Some library might want to generate events in their ctor/dtor.  If
LTTng initialize/finalize its tracepoints/events at the wrong time,
events are lost.

Order of execution of the ctor/dtor is determined by priority.  When
some priorities are equal, the order of execution seems to be
determined by:

	   a) Order of appearance if in the same compilation unit

	   b) Order of link if in different compilation units

	   c) Order of load by ld-linux.so or dlopen(3) for
	      share objects

Also, using the constructor/destructor attributes without any priority
will default to the _lowest_ priority 65535, at least for GCC.

Thus, Providing the LTTNG_*_PRIO definitions allows users to set their
ctor/dtor priority like so:
----------------------------------------------------------------------
...
__attribute__((constructor(LTTNG_CTOR_PRIO + 1)))
...
__attribute__((destructor(LTTNG_DTOR_PRIO + 1)))
...
----------------------------------------------------------------------
or without any priority, that would also work.

Note that LTTNG_*_PRIO are set to 101 because it is the _highest_
priority and 0 to 100 are reserved.

Signed-off-by: Olivier Dion <olivier.dion@polymtl.ca>
---
 include/lttng/tracepoint.h           | 8 ++++----
 include/lttng/ust-compiler.h         | 7 +++++++
 include/lttng/ust-tracepoint-event.h | 4 ++--
 3 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index d77a2fb2..70903757 100644
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -318,7 +318,7 @@ __tracepoint__init_urcu_sym(void)
 }
 #endif
 
-static void lttng_ust_notrace __attribute__((constructor))
+static void lttng_ust_notrace __attribute__((constructor(LTTNG_CTOR_PRIO)))
 __tracepoints__init(void);
 static void
 __tracepoints__init(void)
@@ -340,7 +340,7 @@ __tracepoints__init(void)
 	__tracepoint__init_urcu_sym();
 }
 
-static void lttng_ust_notrace __attribute__((destructor))
+static void lttng_ust_notrace __attribute__((destructor(LTTNG_DTOR_PRIO)))
 __tracepoints__destroy(void);
 static void
 __tracepoints__destroy(void)
@@ -444,7 +444,7 @@ extern struct lttng_ust_tracepoint * const __stop___tracepoints_ptrs[]
 		__attribute__((used, section("__tracepoints_ptrs"))) =		\
 			&__tracepoint_##_provider##___##_name;
 
-static void lttng_ust_notrace __attribute__((constructor))
+static void lttng_ust_notrace __attribute__((constructor(LTTNG_CTOR_PRIO)))
 __tracepoints__ptrs_init(void);
 static void
 __tracepoints__ptrs_init(void)
@@ -488,7 +488,7 @@ __tracepoints__ptrs_init(void)
 	}
 }
 
-static void lttng_ust_notrace __attribute__((destructor))
+static void lttng_ust_notrace __attribute__((destructor(LTTNG_DTOR_PRIO)))
 __tracepoints__ptrs_destroy(void);
 static void
 __tracepoints__ptrs_destroy(void)
diff --git a/include/lttng/ust-compiler.h b/include/lttng/ust-compiler.h
index 1d04da1a..4f1b1a40 100644
--- a/include/lttng/ust-compiler.h
+++ b/include/lttng/ust-compiler.h
@@ -27,4 +27,11 @@
 #define lttng_ust_notrace __attribute__((no_instrument_function))
 #define LTTNG_PACKED	__attribute__((__packed__))
 
+/*
+ * Value in range [0, 100] are reserved.  Thus 101 is the higest
+ * priority that can be used.
+ */
+#define LTTNG_CTOR_PRIO 101
+#define LTTNG_DTOR_PRIO 101
+
 #endif /* _LTTNG_UST_COMPILER_H */
diff --git a/include/lttng/ust-tracepoint-event.h b/include/lttng/ust-tracepoint-event.h
index 7890c247..cd45ae08 100644
--- a/include/lttng/ust-tracepoint-event.h
+++ b/include/lttng/ust-tracepoint-event.h
@@ -1009,7 +1009,7 @@ static int _TP_COMBINE_TOKENS(__probe_register_refcount___, TRACEPOINT_PROVIDER)
 
 /* Reset all macros within TRACEPOINT_EVENT */
 #include <lttng/ust-tracepoint-event-reset.h>
-static void lttng_ust_notrace __attribute__((constructor))
+static void lttng_ust_notrace __attribute__((constructor(LTTNG_CTOR_PRIO)))
 _TP_COMBINE_TOKENS(__lttng_events_init__, TRACEPOINT_PROVIDER)(void);
 static void
 _TP_COMBINE_TOKENS(__lttng_events_init__, TRACEPOINT_PROVIDER)(void)
@@ -1036,7 +1036,7 @@ _TP_COMBINE_TOKENS(__lttng_events_init__, TRACEPOINT_PROVIDER)(void)
 	}
 }
 
-static void lttng_ust_notrace __attribute__((destructor))
+static void lttng_ust_notrace __attribute__((destructor(LTTNG_DTOR_PRIO)))
 _TP_COMBINE_TOKENS(__lttng_events_exit__, TRACEPOINT_PROVIDER)(void);
 static void
 _TP_COMBINE_TOKENS(__lttng_events_exit__, TRACEPOINT_PROVIDER)(void)
-- 
2.27.0