diff -r 1af044412894 libgo/Makefile.am
--- a/libgo/Makefile.am	Wed May 16 07:51:35 2012 -0700
+++ b/libgo/Makefile.am	Wed May 16 22:06:18 2012 -0700
@@ -439,6 +439,7 @@
 	runtime/go-string-to-int-array.c \
 	runtime/go-strplus.c \
 	runtime/go-strslice.c \
+	runtime/go-traceback.c \
 	runtime/go-trampoline.c \
 	runtime/go-type-complex.c \
 	runtime/go-type-eface.c \
diff -r 1af044412894 libgo/runtime/go-signal.c
--- a/libgo/runtime/go-signal.c	Wed May 16 07:51:35 2012 -0700
+++ b/libgo/runtime/go-signal.c	Wed May 16 22:06:18 2012 -0700
@@ -157,7 +157,6 @@
 
   for (i = 0; runtime_sigtab[i].sig != -1; ++i)
     {
-      struct sigaction sa;
       SigTab *t;
 
       t = &runtime_sigtab[i];
@@ -177,21 +176,33 @@
 
       runtime_startpanic ();
 
-      /* We should do a stack backtrace here.  Until we can do that,
-	 we reraise the signal in order to get a slightly better
-	 report from the shell.  */
+      {
+	const char *name = NULL;
 
-      memset (&sa, 0, sizeof sa);
+#ifdef HAVE_STRSIGNAL
+	name = strsignal (sig);
+#endif
 
-      sa.sa_handler = SIG_DFL;
+	if (name == NULL)
+	  runtime_printf ("Signal %d\n", sig);
+	else
+	  runtime_printf ("%s\n", name);
+      }
 
-      i = sigemptyset (&sa.sa_mask);
-      __go_assert (i == 0);
+      runtime_printf ("\n");
 
-      if (sigaction (sig, &sa, NULL) != 0)
-	abort ();
+      if (runtime_gotraceback ())
+	{
+	  G *g;
 
-      raise (sig);
+	  g = runtime_g ();
+	  runtime_traceback (g);
+	  runtime_tracebackothers (g);
+
+	  /* The gc library calls runtime_dumpregs here, and provides
+	     a function that prints the registers saved in context in
+	     a readable form.  */
+	}
 
       runtime_exit (2);
     }
@@ -230,12 +241,22 @@
 sig_panic_info_handler (int sig, siginfo_t *info,
 			void *context __attribute__ ((unused)))
 {
-  if (runtime_g () == NULL || info->si_code == SI_USER)
+  G *g;
+
+  g = runtime_g ();
+  if (g == NULL || info->si_code == SI_USER)
     {
       sig_handler (sig);
       return;
     }
 
+  g->sig = sig;
+  g->sigcode0 = info->si_code;
+  g->sigcode1 = (uintptr_t) info->si_addr;
+
+  /* It would be nice to set g->sigpc here as the gc library does, but
+     I don't know how to get it portably.  */
+
   sig_panic_leadin (sig);
 
   switch (sig)
@@ -284,12 +305,19 @@
 static void
 sig_panic_handler (int sig)
 {
-  if (runtime_g () == NULL)
+  G *g;
+
+  g = runtime_g ();
+  if (g == NULL)
     {
       sig_handler (sig);
       return;
     }
 
+  g->sig = sig;
+  g->sigcode0 = 0;
+  g->sigcode1 = 0;
+
   sig_panic_leadin (sig);
 
   switch (sig)
diff -r 1af044412894 libgo/runtime/go-traceback.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgo/runtime/go-traceback.c	Wed May 16 22:06:18 2012 -0700
@@ -0,0 +1,62 @@
+/* go-callers.c -- get callers for Go.
+
+   Copyright 2012 The Go Authors. All rights reserved.
+   Use of this source code is governed by a BSD-style
+   license that can be found in the LICENSE file.  */
+
+#include "config.h"
+
+#include "unwind.h"
+
+#include "runtime.h"
+#include "go-string.h"
+
+static _Unwind_Reason_Code
+traceback (struct _Unwind_Context *context, void *varg)
+{
+  int *parg = (int *) varg;
+  uintptr pc;
+  int ip_before_insn = 0;
+  struct __go_string fn;
+  struct __go_string file;
+  int line;
+
+#ifdef HAVE_GETIPINFO
+  pc = _Unwind_GetIPInfo (context, &ip_before_insn);
+#else
+  pc = _Unwind_GetIP (context);
+#endif
+
+  if (*parg > 100)
+    return _URC_END_OF_STACK;
+  ++*parg;
+
+  /* FIXME: If PC is in the __morestack routine, we should ignore
+     it.  */
+
+  /* Back up to the call instruction.  */
+  if (!ip_before_insn)
+    --pc;
+
+  if (!__go_file_line (pc, &fn, &file, &line))
+    return _URC_END_OF_STACK;
+
+  if (runtime_showframe (fn.__data))
+    {
+      runtime_printf ("%s\n", fn.__data);
+      runtime_printf ("\t%s:%d\n", file.__data, line);
+    }
+
+  return _URC_NO_REASON;
+}
+
+/* Print a stack trace for the current goroutine.  */
+
+void
+runtime_traceback ()
+{
+  int c;
+
+  c = 0;
+  _Unwind_Backtrace (traceback, &c);
+}
diff -r 1af044412894 libgo/runtime/mprof.goc
--- a/libgo/runtime/mprof.goc	Wed May 16 07:51:35 2012 -0700
+++ b/libgo/runtime/mprof.goc	Wed May 16 22:06:18 2012 -0700
@@ -361,10 +361,11 @@
 		g->writenbuf = b.__count;
 		USED(pc);
 		USED(sp);
-		// runtime_goroutineheader(g);
-		// runtime_traceback(pc, sp, 0, g);
-		// if(all)
-		//	runtime_tracebackothers(g);
+		runtime_goroutineheader(g);
+		runtime_traceback();
+		runtime_goroutinetrailer(g);
+		if(all)
+			runtime_tracebackothers(g);
 		n = b.__count - g->writenbuf;
 		g->writebuf = nil;
 		g->writenbuf = 0;
diff -r 1af044412894 libgo/runtime/proc.c
--- a/libgo/runtime/proc.c	Wed May 16 07:51:35 2012 -0700
+++ b/libgo/runtime/proc.c	Wed May 16 22:06:18 2012 -0700
@@ -50,6 +50,8 @@
 
 static void schedule(G*);
 
+static void gtraceback(G*);
+
 typedef struct Sched Sched;
 
 M	runtime_m0;
@@ -345,6 +347,9 @@
 		// the values for this thread.
 		mp = runtime_m();
 		gp = runtime_g();
+
+		if(gp->dotraceback != nil)
+			gtraceback(gp);
 	}
 	if (gp == nil || !gp->fromgogo) {
 #ifdef USING_SPLIT_STACK
@@ -523,19 +528,73 @@
 }
 
 void
-runtime_tracebackothers(G *me)
+runtime_goroutinetrailer(G *g)
 {
-	G *g;
+	if(g != nil && g->gopc != 0 && g->goid != 1) {
+		struct __go_string fn;
+		struct __go_string file;
+		int line;
+
+		if(__go_file_line(g->gopc - 1, &fn, &file, &line)) {
+			runtime_printf("created by %s\n", fn.__data);
+			runtime_printf("\t%s:%d\n", file.__data, line);
+		}
+	}
+}
+
+void
+runtime_tracebackothers(G * volatile me)
+{
+	G * volatile g;
 
 	for(g = runtime_allg; g != nil; g = g->alllink) {
 		if(g == me || g->status == Gdead)
 			continue;
 		runtime_printf("\n");
 		runtime_goroutineheader(g);
-		// runtime_traceback(g->sched.pc, g->sched.sp, 0, g);
+
+		// Our only mechanism for doing a stack trace is
+		// _Unwind_Backtrace.  And that only works for the
+		// current thread, not for other random goroutines.
+		// So we need to switch context to the goroutine, get
+		// the backtrace, and then switch back.
+
+		// This means that if is running or in a syscall, we
+		// can't reliably print a stack trace.  FIXME.
+		if(g->status == Gsyscall || g->status == Grunning) {
+			runtime_printf("no stack trace available\n");
+			runtime_goroutinetrailer(g);
+			continue;
+		}
+
+		g->dotraceback = me;
+
+#ifdef USING_SPLIT_STACK
+		__splitstack_getcontext(&me->stack_context[0]);
+#endif
+		getcontext(&me->context);
+
+		if(g->dotraceback) {
+			runtime_gogo(g);
+		}
 	}
 }
 
+// Do a stack trace of gp, and then restore the context to
+// g->dotraceback.
+
+static void
+gtraceback(G* gp)
+{
+	G* ret;
+
+	runtime_traceback(nil);
+	runtime_goroutinetrailer(gp);
+	ret = gp->dotraceback;
+	gp->dotraceback = nil;
+	runtime_gogo(ret);
+}
+
 // Mark this g as m's idle goroutine.
 // This functionality might be used in environments where programs
 // are limited to a single thread, to simulate a select-driven
@@ -1171,7 +1230,7 @@
 
 	// Leave SP around for gc and traceback.
 #ifdef USING_SPLIT_STACK
-	g->gcstack = __splitstack_find(NULL, NULL, &g->gcstack_size,
+	g->gcstack = __splitstack_find(nil, nil, &g->gcstack_size,
 				       &g->gcnext_segment, &g->gcnext_sp,
 				       &g->gcinitial_sp);
 #else
@@ -1227,9 +1286,11 @@
 	// find that we still have mcpu <= mcpumax, then we can
 	// start executing Go code immediately, without having to
 	// schedlock/schedunlock.
+	// Also do fast return if any locks are held, so that
+	// panic code can use syscalls to open a file.
 	gp = g;
 	v = runtime_xadd(&runtime_sched.atomic, (1<<mcpuShift));
-	if(m->profilehz == runtime_sched.profilehz && atomic_mcpu(v) <= atomic_mcpumax(v)) {
+	if((m->profilehz == runtime_sched.profilehz && atomic_mcpu(v) <= atomic_mcpumax(v)) || m->locks > 0) {
 		// There's a cpu for us, so we can run.
 		gp->status = Grunning;
 		// Garbage collector isn't running (since we are),
@@ -1561,7 +1622,7 @@
 		uint8 *lr __attribute__ ((unused)),
 		G *gp __attribute__ ((unused)))
 {
-	// int32 n;
+	int32 n;
 
 	if(prof.fn == nil || prof.hz == 0)
 		return;
@@ -1571,9 +1632,9 @@
 		runtime_unlock(&prof);
 		return;
 	}
-	// n = runtime_gentraceback(pc, sp, lr, gp, 0, prof.pcbuf, nelem(prof.pcbuf));
-	// if(n > 0)
-	// 	prof.fn(prof.pcbuf, n);
+	n = runtime_callers(0, prof.pcbuf, nelem(prof.pcbuf));
+	if(n > 0)
+		prof.fn(prof.pcbuf, n);
 	runtime_unlock(&prof);
 }
 
diff -r 1af044412894 libgo/runtime/runtime.c
--- a/libgo/runtime/runtime.c	Wed May 16 07:51:35 2012 -0700
+++ b/libgo/runtime/runtime.c	Wed May 16 22:06:18 2012 -0700
@@ -11,6 +11,17 @@
 
 uint32	runtime_panicking;
 
+int32
+runtime_gotraceback(void)
+{
+	const byte *p;
+
+	p = runtime_getenv("GOTRACEBACK");
+	if(p == nil || p[0] == '\0')
+		return 1;	// default is on
+	return runtime_atoi(p);
+}
+
 static Lock paniclk;
 
 void
@@ -31,20 +42,26 @@
 void
 runtime_dopanic(int32 unused __attribute__ ((unused)))
 {
-	/*
+	G* g;
 	static bool didothers;
 
+	g = runtime_g();
 	if(g->sig != 0)
-		runtime_printf("[signal %x code=%p addr=%p pc=%p]\n",
-			g->sig, g->sigcode0, g->sigcode1, g->sigpc);
+		runtime_printf("[signal %x code=%p addr=%p]\n",
+			g->sig, (void*)(g->sigcode0), (void*)(g->sigcode1));
 
 	if(runtime_gotraceback()){
+		if(g != runtime_m()->g0) {
+			runtime_printf("\n");
+			runtime_goroutineheader(g);
+			runtime_traceback();
+			runtime_goroutinetrailer(g);
+		}
 		if(!didothers) {
 			didothers = true;
 			runtime_tracebackothers(g);
 		}
 	}
-	*/
 
 	runtime_unlock(&paniclk);
 	if(runtime_xadd(&runtime_panicking, -1) != 0) {
@@ -185,10 +202,10 @@
 }
 
 static struct root_list runtime_roots =
-{ NULL,
+{ nil,
   { { &syscall_Envs, sizeof syscall_Envs },
     { &os_Args, sizeof os_Args },
-    { NULL, 0 } },
+    { nil, 0 } },
 };
 
 void
@@ -209,3 +226,13 @@
   return 0;
 #endif
 }
+
+bool
+runtime_showframe(const unsigned char *s)
+{
+	static int32 traceback = -1;
+	
+	if(traceback < 0)
+		traceback = runtime_gotraceback();
+	return traceback > 1 || (__builtin_strchr((const char*)s, '.') != nil && __builtin_memcmp(s, "runtime.", 7) != 0);
+}
diff -r 1af044412894 libgo/runtime/runtime.h
--- a/libgo/runtime/runtime.h	Wed May 16 07:51:35 2012 -0700
+++ b/libgo/runtime/runtime.h	Wed May 16 22:06:18 2012 -0700
@@ -143,14 +143,16 @@
 	M*	m;		// for debuggers, but offset not hard-coded
 	M*	lockedm;
 	M*	idlem;
-	// int32	sig;
+	int32	sig;
 	int32	writenbuf;
 	byte*	writebuf;
-	// uintptr	sigcode0;
-	// uintptr	sigcode1;
+	uintptr	sigcode0;
+	uintptr	sigcode1;
 	// uintptr	sigpc;
 	uintptr	gopc;	// pc of go statement that created this goroutine
 
+	G*	dotraceback;
+
 	ucontext_t	context;
 	void*		stack_context[10];
 };
@@ -289,6 +291,11 @@
 void	runtime_schedinit(void);
 void	runtime_initsig(void);
 void	runtime_sigenable(uint32 sig);
+int32	runtime_gotraceback(void);
+void	runtime_goroutineheader(G*);
+void	runtime_goroutinetrailer(G*);
+void	runtime_traceback();
+void	runtime_tracebackothers(G*);
 String	runtime_gostringnocopy(const byte*);
 void*	runtime_mstart(void*);
 G*	runtime_malg(int32, byte**, size_t*);
@@ -434,6 +441,8 @@
 void	runtime_LockOSThread(void) __asm__("runtime.LockOSThread");
 void	runtime_UnlockOSThread(void) __asm__("runtime.UnlockOSThread");
 
+bool	runtime_showframe(const unsigned char*);
+
 uintptr	runtime_memlimit(void);
 
 // If appropriate, ask the operating system to control whether this
@@ -468,3 +477,5 @@
 // This will be 0 when using split stacks, as in that case
 // the stacks are allocated by the splitstack library.
 extern uintptr runtime_stacks_sys;
+
+extern _Bool __go_file_line (uintptr, String*, String*, int *);
