Avoid crash when M structure free just before thread exit.

From-SVN: r169121

libgo/runtime/go-go.c

@@ -94,6 +94,13 @@ remove_current_thread (void)
 
   runtime_MCache_ReleaseAll (mcache);
 
+  /* As soon as we release this look, a GC could run.  Since this
+     thread is no longer on the list, the GC will not find our M
+     structure, so it could get freed at any time.  That means that
+     any code from here to thread exit must not assume that the m is
+     valid.  */
+  m = NULL;
+
   i = pthread_mutex_unlock (&__go_thread_ids_lock);
   __go_assert (i == 0);
 

libgo/runtime/thread.c

@@ -38,9 +38,11 @@ runtime_lock_full(Lock *l)
 void
 runtime_lock(Lock *l)
 {
-	if(m->locks < 0)
-		runtime_throw("lock count");
-	m->locks++;
+	if(m != nil) {
+		if(m->locks < 0)
+			runtime_throw("lock count");
+		m->locks++;
+	}
 
 	if(runtime_xadd(&l->key, 1) > 1)	// someone else has it; wait
 		runtime_lock_full(l);
@@ -58,9 +60,11 @@ runtime_unlock_full(Lock *l)
 void
 runtime_unlock(Lock *l)
 {
-	m->locks--;
-	if(m->locks < 0)
-		runtime_throw("lock count");
+	if(m != nil) {
+		m->locks--;
+		if(m->locks < 0)
+			runtime_throw("lock count");
+	}
 
 	if(runtime_xadd(&l->key, -1) > 0)	// someone else is waiting
 		runtime_unlock_full(l);