Interlocks sckbdevent() and sccngetch() with the low-level atomic
semaphores.

--- sys/dev/syscons/syscons.c.orig	2008-08-31 14:17:40.000000000 +0400
+++ sys/dev/syscons/syscons.c	2008-11-24 08:39:07.000000000 +0300
@@ -60,6 +60,7 @@
 #include <sys/tty.h>
 #include <sys/power.h>
 
+#include <machine/atomic.h>
 #include <machine/clock.h>
 #if defined(__sparc64__) || defined(__powerpc__)
 #include <machine/sc_machdep.h>
@@ -165,6 +166,13 @@
 
 static	int		debugger;
 
+/*
+ * Semaphore that uses for locking within syscons.  As was noted by
+ * Bruce Evans, locks can't be accessed in the debugger mode, but
+ * debugger uses keyboard sometimes ;))
+ */
+static	int		_syscons_semaphore;
+
 /* prototypes */
 static int sc_allocate_keyboard(sc_softc_t *sc, int unit);
 static int scvidprobe(int unit, int flags, int cons);
@@ -609,7 +617,7 @@
 {
     sc_softc_t *sc;
     struct tty *cur_tty;
-    int c, error = 0; 
+    int c, error = 0, sem_owned = 0;
     size_t len;
     u_char *cp;
 
@@ -631,12 +639,20 @@
 	goto done;
     }
 
+    /*
+     * Grab semaphore to notify others that we're doing our job.
+     */
+    sem_owned = atomic_cmpset_acq_int(&_syscons_semaphore, 0, 1);
+
     /* 
      * Loop while there is still input to get from the keyboard.
      * I don't think this is nessesary, and it doesn't fix
      * the Xaccel-2.1 keyboard hang, but it can't hurt.		XXX
+     *
+     * If semaphore is already grabbed, don't poll.
      */
-    while ((c = scgetc(sc, SCGETC_NONBLOCK)) != NOKEY) {
+    while (sem_owned &&
+        (c = scgetc(sc, SCGETC_NONBLOCK)) != NOKEY) {
 
 	cur_tty = SC_DEV(sc, sc->cur_scp->index);
 	if (!tty_opened(cur_tty))
@@ -677,6 +693,8 @@
     sc->cur_scp->status |= MOUSE_HIDDEN;
 
 done:
+    if (sem_owned)
+	atomic_clear_int(&_syscons_semaphore, 1);
     mtx_unlock(&Giant);
     return (error);
 }
@@ -1569,10 +1587,10 @@
     scr_stat *scp;
     u_char *p;
     int cur_mode;
-    int s = spltty();	/* block sckbdevent and scrn_timer while we poll */
     int c;
 
-    /* assert(sc_console != NULL) */
+    if (!atomic_cmpset_acq_int(&_syscons_semaphore, 0, 1))
+	return -1;
 
     /* 
      * Stop the screen saver and update the screen if necessary.
@@ -1583,12 +1601,13 @@
     sccnupdate(scp);
 
     if (fkeycp < fkey.len) {
-	splx(s);
-	return fkey.str[fkeycp++];
+	c = fkey.str[fkeycp++];
+	atomic_clear_int(&_syscons_semaphore, 1);
+	return c;
     }
 
     if (scp->sc->kbd == NULL) {
-	splx(s);
+	atomic_clear_int(&_syscons_semaphore, 1);
 	return -1;
     }
 
@@ -1610,26 +1629,29 @@
     scp->kbd_mode = cur_mode;
     kbdd_ioctl(scp->sc->kbd, KDSKBMODE, (caddr_t)&scp->kbd_mode);
     kbdd_disable(scp->sc->kbd);
-    splx(s);
 
     switch (KEYFLAGS(c)) {
     case 0:	/* normal char */
-	return KEYCHAR(c);
+	c = KEYCHAR(c);
+	break;
     case FKEY:	/* function key */
 	p = kbdd_get_fkeystr(scp->sc->kbd, KEYCHAR(c), (size_t *)&fkeycp);
 	fkey.len = fkeycp;
 	if ((p != NULL) && (fkey.len > 0)) {
 	    bcopy(p, fkey.str, fkey.len);
 	    fkeycp = 1;
-	    return fkey.str[0];
+	    c = fkey.str[0];
 	}
-	return c;	/* XXX */
+	/* XXX */
+	break;
     case NOKEY:
     case ERRKEY:
     default:
-	return -1;
+	c = -1;
+	break;
     }
-    /* NOT REACHED */
+    atomic_clear_int(&_syscons_semaphore, 1);
+    return c;
 }
 
 static void