Index: head/sys/kern/kern_fork.c
===================================================================
--- head/sys/kern/kern_fork.c
+++ head/sys/kern/kern_fork.c
@@ -586,7 +586,7 @@
 	 * been preserved.
 	 */
 	p2->p_flag |= p1->p_flag & P_SUGID;
-	td2->td_pflags |= td->td_pflags & TDP_ALTSTACK;
+	td2->td_pflags |= (td->td_pflags & TDP_ALTSTACK) | TDP_FORKING;
 	SESS_LOCK(p1->p_session);
 	if (p1->p_session->s_ttyvp != NULL && p1->p_flag & P_CONTROLT)
 		p2->p_flag |= P_CONTROLT;
@@ -1023,6 +1023,7 @@
 
 	if (p->p_sysent->sv_schedtail != NULL)
 		(p->p_sysent->sv_schedtail)(td);
+	td->td_pflags &= ~TDP_FORKING;
 }
 
 /*
Index: head/sys/kern/subr_syscall.c
===================================================================
--- head/sys/kern/subr_syscall.c
+++ head/sys/kern/subr_syscall.c
@@ -176,6 +176,9 @@
 	struct proc *p, *p2;
 	int traced;
 
+	KASSERT((td->td_pflags & TDP_FORKING) == 0,
+	    ("fork() did not clear TDP_FORKING upon completion"));
+
 	p = td->td_proc;
 
 	/*
Index: head/sys/sys/proc.h
===================================================================
--- head/sys/sys/proc.h
+++ head/sys/sys/proc.h
@@ -446,7 +446,7 @@
 #define	TDP_RESETSPUR	0x04000000 /* Reset spurious page fault history. */
 #define	TDP_NERRNO	0x08000000 /* Last errno is already in td_errno */
 #define	TDP_UIOHELD	0x10000000 /* Current uio has pages held in td_ma */
-#define	TDP_UNUSED29	0x20000000 /* --available-- */
+#define	TDP_FORKING	0x20000000 /* Thread is being created through fork() */
 #define	TDP_EXECVMSPC	0x40000000 /* Execve destroyed old vmspace */
 
 /*