[profiler] Load the profiler with MONO_DL_EAGER.

We need to make sure all external references have been resolved so that we
don't invoke the dynamic linker in the mono_sample_hit () callback which is
called in async-signal context.

mono/metadata/profiler.c

@@ -1147,7 +1147,15 @@ load_embedded_profiler (const char *desc, const char *name)
 	MonoDl *pmodule = NULL;
 	gboolean result;
 
-	pmodule = mono_dl_open (NULL, MONO_DL_LAZY, &err);
+	/*
+	 * Some profilers (such as ours) may need to call back into the runtime
+	 * from their sampling callback (which is called in async-signal context).
+	 * They need to be able to know that all references back to the runtime
+	 * have been resolved; otherwise, calling runtime functions may result in
+	 * invoking the dynamic linker which is not async-signal-safe. Passing
+	 * MONO_DL_EAGER will ask the dynamic linker to resolve everything upfront.
+	 */
+	pmodule = mono_dl_open (NULL, MONO_DL_EAGER, &err);
 	if (!pmodule) {
 		g_warning ("Could not open main executable (%s)", err);
 		g_free (err);
@@ -1175,7 +1183,7 @@ load_profiler_from_directory (const char *directory, const char *libname, const
 	iter = NULL;
 	err = NULL;
 	while ((path = mono_dl_build_path (directory, libname, &iter))) {
-		pmodule = mono_dl_open (path, MONO_DL_LAZY, &err);
+		pmodule = mono_dl_open (path, MONO_DL_EAGER, &err);
 		mono_trace (G_LOG_LEVEL_INFO, MONO_TRACE_DLLIMPORT, "Attempting to load profiler: %s, %ssuccessful, err: %s", path, pmodule?"":"not ", err);
 		g_free (path);
 		g_free (err);
@@ -1190,7 +1198,7 @@ static gboolean
 load_profiler_from_mono_installation (const char *libname, const char *desc)
 {
 	char *err = NULL;
-	MonoDl *pmodule = mono_dl_open_runtime_lib (libname, MONO_DL_LAZY, &err);
+	MonoDl *pmodule = mono_dl_open_runtime_lib (libname, MONO_DL_EAGER, &err);
 	mono_trace (G_LOG_LEVEL_INFO, MONO_TRACE_DLLIMPORT, "Attempting to load profiler from runtime libs: %s, %ssuccessful, err: %s", libname, pmodule?"":"not ", err);
 	g_free (err);
 	if (pmodule)

mono/profiler/proflog.c

@@ -2063,6 +2063,14 @@ enqueue_sample_hit (gpointer p)
 static void
 mono_sample_hit (MonoProfiler *profiler, unsigned char *ip, void *context)
 {
+	/*
+	 * Please note: We rely on the runtime loading the profiler with
+	 * MONO_DL_EAGER (RTLD_NOW) so that references to runtime functions within
+	 * this function (and its siblings) are resolved when the profiler is
+	 * loaded. Otherwise, we would potentially invoke the dynamic linker when
+	 * invoking runtime functions, which is not async-signal-safe.
+	 */
+
 	if (in_shutdown)
 		return;