9736 kmdb tortures via single-step miscellaneous trap
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>

@@ -19,14 +19,13 @@
  * CDDL HEADER END
  */
 /*
  * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
+ * Copyright 2018 Joyent, Inc.
  */
 
-#pragma ident   "%Z%%M% %I%     %E% SMI"
-
 /*
  * The main CPU-control loops, used to control masters and slaves.
  */
 
 #include <sys/types.h>

@@ -290,17 +289,35 @@
 kaif_main_loop(kaif_cpusave_t *cpusave)
 {
         int cmd;
 
         if (kaif_master_cpuid == KAIF_MASTER_CPUID_UNSET) {
+
+                /*
+                 * Special case: Unload requested before first debugger entry.
+                 * Don't stop the world, as there's nothing to clean up that
+                 * can't be handled by the running kernel.
+                 */
                 if (!kmdb_dpi_resume_requested &&
                     kmdb_kdi_get_unload_request()) {
+                        cpusave->krs_cpu_state = KAIF_CPU_STATE_NONE;
+                        return (KAIF_CPU_CMD_RESUME);
+                }
+
                         /*
-                         * Special case: Unload requested before first debugger
-                         * entry.  Don't stop the world, as there's nothing to
-                         * clean up that can't be handled by the running kernel.
+                 * We're a slave with no master, so just resume.  This can
+                 * happen if, prior to this, two CPUs both raced through
+                 * kdi_cmnint() - for example, a breakpoint on a frequently
+                 * called function.  The loser will be redirected to the slave
+                 * loop; note that the event itself is lost at this point.
+                 *
+                 * The winner will then cross-call that slave, but it won't
+                 * actually be received until the slave returns to the kernel
+                 * and enables interrupts.  We'll then come back in via
+                 * kdi_slave_entry() and hit this path.
                          */
+                if (cpusave->krs_cpu_state == KAIF_CPU_STATE_SLAVE) {
                         cpusave->krs_cpu_state = KAIF_CPU_STATE_NONE;
                         return (KAIF_CPU_CMD_RESUME);
                 }
 
                 kaif_select_master(cpusave);