panic: mi_switch: did not reenter debugger

This panic happens often when setting a breakpoint and then hitting
it a few times.  I think I see the problem.

The bsd code in kern/subr_kdb.c does this:


#ifdef SMP
        if (did_stop_cpus)
                restart_cpus(stopped_cpus);
#endif

        kdb_active--;


The panic is this (mi_switch):

        /*
         * Don't perform context switches from the debugger.
         */
        if (kdb_active) {
                mtx_unlock_spin(&sched_lock);
                kdb_backtrace();
                kdb_reenter();
                panic("%s: did not reenter debugger", __func__);
        }


It's an mp-race.  It restarts the other CPU's before decrementing
kdb_active.   If the other CPU races into mi_switch first, a
panic is guaranteed.

Fix: 

Decrement kdb_active before restarting cpus.
How-To-Repeat:       Hit the same breakpoint several times on an SMP box.