=== modified file 'doc/emacs/trouble.texi'
--- doc/emacs/trouble.texi	2011-01-25 04:08:28 +0000
+++ doc/emacs/trouble.texi	2011-03-28 06:53:07 +0000
@@ -812,6 +812,15 @@
 This backtrace is useful for debugging such long loops, so if you can
 produce it, copy it into the bug report.
 
address@hidden sigusr2-action
+If normal quitting does nothing and you are using a system that
+supports signals, then as a last resort, you can try sending Emacs the
+SIGUSR2 signal.  When Emacs receives this signal, it checks whether
address@hidden is equal to @code{quit}.  If so, Emacs stops
+what it's doing, sets @code{debug-on-quit} to @code{t}, and quits just
+as if you had typed @kbd{C-g}.  This process happens even in places
+where quitting is normally not allowed.
+
 @item
 Check whether any programs you have loaded into the Lisp world,
 including your @file{.emacs} file, set any variables that may affect the

=== modified file 'doc/lispref/debugging.texi'
--- doc/lispref/debugging.texi	2011-01-25 04:08:28 +0000
+++ doc/lispref/debugging.texi	2011-03-28 06:54:07 +0000
@@ -185,6 +185,19 @@
 when you quit.  @xref{Quitting}.
 @end defopt
 
address@hidden sigusr2-action
+On systems with signal support, this variable determines what happens
+when Emacs receives SIGUSR2.  When its value is @code{quit}, Emacs
+will quit just as if @kbd{C-g} had been pressed, except that Emacs
+will first set @code{debug-on-quit} to @code{t} and
address@hidden to @code{nil} so that the quit is not ignored.
+When @code{sigusr2-action} has any other value, Emacs will handle the
+signal as specified in @code{special-event-map} (@pxref{Active
+Keymaps}).  This feature is useful for terminating infinite loops in
+places where quits would normally be ignored, such as in code that
+runs during redisplay.
address@hidden defopt
+
 @node Function Debugging
 @subsection Entering the Debugger on a Function Call
 @cindex function call debugging

=== modified file 'lisp/cus-start.el'
--- lisp/cus-start.el	2011-03-27 10:55:07 +0000
+++ lisp/cus-start.el	2011-03-28 06:04:18 +0000
@@ -259,6 +259,10 @@
 	     (suggest-key-bindings keyboard (choice (const :tag "off" nil)
 						    (integer :tag "time" 2)
 						    (other :tag "on")))
+             (sigusr2-action debug
+			    (choice (const :tag "Handle SIGUSR2 normally")
+                                    (const :tag "Quit on SIGUSR2" quit))
+                            "24.1")
 
 ;; This is not good news because it will use the wrong
 ;; version-specific directories when you upgrade.  We need

=== modified file 'src/keyboard.c'
--- src/keyboard.c	2011-03-27 02:27:11 +0000
+++ src/keyboard.c	2011-03-28 06:46:50 +0000
@@ -7168,6 +7168,21 @@
 
   SIGNAL_THREAD_CHECK (sig);
 
+#ifdef SIGUSR2
+  if (sig == SIGUSR2)
+    {
+      /* We can be called at any time, but because variable assignment
+         is atomic, any races are harmless.  */
+      if (Vsigusr2_action == Qquit)
+        {
+          debug_on_quit = 1;
+          Vquit_flag = Qt;
+          Vinhibit_quit = Qnil;
+          return;
+        }
+    }
+#endif /* SIGUSR2 */
+
   for (p = user_signals; p; p = p->next)
     if (p->sig == sig)
       {
@@ -11356,6 +11371,10 @@
   poll_suppress_count = 1;
   start_polling ();
 #endif
+
+#ifdef SIGUSR2
+  Vsigusr2_action = Qquit;
+#endif
 }
 
 /* This type's only use is in syms_of_keyboard, to initialize the
@@ -12178,6 +12197,19 @@
 `deactivate-mark' call uses this to set the window selection.  */);
   Vsaved_region_selection = Qnil;
 
+#ifdef SIGUSR2
+  DEFVAR_LISP ("sigusr2-action",
+               Vsigusr2_action,
+               doc: /* Controls what Emacs does when it receives a
+SIGUSR2.  If 'quit, set `inhibit-quit' to nil and `quit-flag' to t,
+quitting unconditionally at the next opportunity. Any other value
+causes nothing to be done.
+
+In all cases, Emacs generates a `sigusr2' event as it normally
+would. See `special-event-map'.  */);
+  Vsigusr2_action = Qnil;
+#endif
+
   /* Create the initial keyboard. */
   initial_kboard = (KBOARD *) xmalloc (sizeof (KBOARD));
   init_kboard (initial_kboard);