- Borrow the KSE single threading code for exec and exit. We use the check

if (p->p_numthreads > 1) and not a flag because action is only necessary
   if there are other threads.  The rest of the system has no need to
   identify thr threaded processes.
 - In kern_thread.c use thr_exit1() instead of thread_exit() if P_THREADED
   is not set.

sys/kern/kern_exec.c

@@ -189,7 +189,7 @@ kern_execve(td, fname, argv, envv, mac_p)
 	PROC_LOCK(p);
 	KASSERT((p->p_flag & P_INEXEC) == 0,
 	    ("%s(): process already has P_INEXEC flag", __func__));
-	if (p->p_flag & P_THREADED) {
+	if (p->p_flag & P_THREADED || p->p_numthreads > 1) {
 		if (thread_single(SINGLE_EXIT)) {
 			PROC_UNLOCK(p);
 			return (ERESTART);	/* Try again later. */

sys/kern/kern_exit.c

@@ -129,15 +129,17 @@ exit1(struct thread *td, int rv)
 	}
 
 	/*
-	 * XXXKSE: MUST abort all other threads before proceeding past here.
+	 * MUST abort all other threads before proceeding past here.
 	 */
 	PROC_LOCK(p);
-	if (p->p_flag & P_THREADED) {
+	if (p->p_flag & P_THREADED || p->p_numthreads > 1) {
 		/*
 		 * First check if some other thread got here before us..
 		 * if so, act apropriatly, (exit or suspend);
 		 */
+		DROP_GIANT();
 		thread_suspend_check(0);
+		PICKUP_GIANT();
 
 		/*
 		 * Kill off the other threads. This requires

sys/kern/kern_fork.c

@@ -265,6 +265,11 @@ fork1(td, flags, pages, procp)
 		return (0);
 	}
 
+	/*
+	 * Note 1:1 allows for forking with one thread coming out on the
+	 * other side with the expectation that the process is about to
+	 * exec.
+	 */
 	if (p1->p_flag & P_THREADED) {
 		/*
 		 * Idle the other threads for a second.

sys/kern/kern_kse.c

@@ -1813,7 +1813,7 @@ thread_single(int force_exit)
 	PROC_LOCK_ASSERT(p, MA_OWNED);
 	KASSERT((td != NULL), ("curthread is NULL"));
 
-	if ((p->p_flag & P_THREADED) == 0)
+	if ((p->p_flag & P_THREADED) == 0 && p->p_numthreads == 1)
 		return (0);
 
 	/* Is someone already single threading? */
@@ -1872,6 +1872,7 @@ thread_single(int force_exit)
 		 * In the mean time we suspend as well.
 		 */
 		thread_suspend_one(td);
+		/* XXX If you recursed this is broken. */
 		mtx_unlock(&Giant);
 		PROC_UNLOCK(p);
 		p->p_stats->p_ru.ru_nvcsw++;
@@ -1961,15 +1962,18 @@ thread_suspend_check(int return_instead)
 		if ((p->p_flag & P_SINGLE_EXIT) && (p->p_singlethread != td)) {
 			while (mtx_owned(&Giant))
 				mtx_unlock(&Giant);
-			thread_exit();
+			if (p->p_flag & P_THREADED)
+				thread_exit();
+			else
+				thr_exit1();
 		}
 
+		mtx_assert(&Giant, MA_NOTOWNED);
 		/*
 		 * When a thread suspends, it just
 		 * moves to the processes's suspend queue
 		 * and stays there.
 		 */
-		mtx_assert(&Giant, MA_NOTOWNED);
 		thread_suspend_one(td);
 		PROC_UNLOCK(p);
 		if (P_SHOULDSTOP(p) == P_STOPPED_SINGLE) {

sys/kern/kern_synch.c

@@ -161,7 +161,7 @@ msleep(ident, mtx, priority, wmesg, timo)
 	 * the thread (recursion here might be bad).
 	 * Hence the TDF_INMSLEEP flag.
 	 */
-	if (p->p_flag & P_THREADED) {
+	if (p->p_flag & P_THREADED || p->p_numthreads > 1) {
 		/*
 		 * Just don't bother if we are exiting
 		 * and not the exiting thread or thread was marked as

sys/kern/kern_thread.c

@@ -1813,7 +1813,7 @@ thread_single(int force_exit)
 	PROC_LOCK_ASSERT(p, MA_OWNED);
 	KASSERT((td != NULL), ("curthread is NULL"));
 
-	if ((p->p_flag & P_THREADED) == 0)
+	if ((p->p_flag & P_THREADED) == 0 && p->p_numthreads == 1)
 		return (0);
 
 	/* Is someone already single threading? */
@@ -1872,6 +1872,7 @@ thread_single(int force_exit)
 		 * In the mean time we suspend as well.
 		 */
 		thread_suspend_one(td);
+		/* XXX If you recursed this is broken. */
 		mtx_unlock(&Giant);
 		PROC_UNLOCK(p);
 		p->p_stats->p_ru.ru_nvcsw++;
@@ -1961,15 +1962,18 @@ thread_suspend_check(int return_instead)
 		if ((p->p_flag & P_SINGLE_EXIT) && (p->p_singlethread != td)) {
 			while (mtx_owned(&Giant))
 				mtx_unlock(&Giant);
-			thread_exit();
+			if (p->p_flag & P_THREADED)
+				thread_exit();
+			else
+				thr_exit1();
 		}
 
+		mtx_assert(&Giant, MA_NOTOWNED);
 		/*
 		 * When a thread suspends, it just
 		 * moves to the processes's suspend queue
 		 * and stays there.
 		 */
-		mtx_assert(&Giant, MA_NOTOWNED);
 		thread_suspend_one(td);
 		PROC_UNLOCK(p);
 		if (P_SHOULDSTOP(p) == P_STOPPED_SINGLE) {