--- src/sys/kern/kern_fork.c	2006/05/24 18:59:48	1.49
+++ src/sys/kern/kern_fork.c	2006/05/29 22:57:22	1.50
@@ -434,11 +434,16 @@ fork1(struct lwp *lp1, int flags, struct
 	 * Inherit the scheduler and initialize scheduler-related fields. 
 	 * Set cpbase to the last timeout that occured (not the upcoming
 	 * timeout).
+	 *
+	 * A critical section is required since a timer IPI can update
+	 * scheduler specific data.
 	 */
+	crit_enter();
 	p2->p_usched = p1->p_usched;
 	lp2->lwp_cpbase = mycpu->gd_schedclock.time -
 			mycpu->gd_schedclock.periodic;
 	p2->p_usched->heuristic_forking(&p1->p_lwp, lp2);
+	crit_exit();
 
 	/*
 	 * This begins the section where we must prevent the parent