libgo: Avoid some cases of getting callers recursively.

Avoids hanging inside older versions of glibc that do not
support recurive calls to dl_iterate_phdr.

From-SVN: r205561

libgo/runtime/go-callers.c

@@ -11,6 +11,13 @@
 #include "runtime.h"
 #include "array.h"
 
+/* This is set to non-zero when calling backtrace_full.  This is used
+   to avoid getting hanging on a recursive lock in dl_iterate_phdr on
+   older versions of glibc when a SIGPROF signal arrives while
+   collecting a backtrace.  */
+
+uint32 runtime_in_callers;
+
 /* Argument passed to callback function.  */
 
 struct callers_data
@@ -111,8 +118,10 @@ runtime_callers (int32 skip, Location *locbuf, int32 m)
   data.skip = skip + 1;
   data.index = 0;
   data.max = m;
+  runtime_xadd (&runtime_in_callers, 1);
   backtrace_full (__go_get_backtrace_state (), 0, callback, error_callback,
 		  &data);
+  runtime_xadd (&runtime_in_callers, -1);
   return data.index;
 }
 

libgo/runtime/proc.c

@@ -2454,6 +2454,15 @@ runtime_sigprof()
 		return;
 	}
 	n = 0;
+
+	if(runtime_atomicload(&runtime_in_callers) > 0) {
+		// If SIGPROF arrived while already fetching runtime
+		// callers we can have trouble on older systems
+		// because the unwind library calls dl_iterate_phdr
+		// which was not recursive in the past.
+		traceback = false;
+	}
+
 	if(traceback) {
 		n = runtime_callers(0, prof.locbuf, nelem(prof.locbuf));
 		for(i = 0; i < n; i++)

libgo/runtime/runtime.h

@@ -776,6 +776,7 @@ extern struct backtrace_state *__go_get_backtrace_state(void);
 extern _Bool __go_file_line(uintptr, String*, String*, intgo *);
 extern byte* runtime_progname();
 extern void runtime_main(void*);
+extern uint32 runtime_in_callers;
 
 int32 getproccount(void);