Add thread_reap_barrier()

Reviewed by:	hselasky,markj
Sponsored by:	Mellanox Technologies/NVidia Networking
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D30468

sys/kern/kern_thread.c

@@ -709,6 +709,38 @@ thread_reap_callout_cb(void *arg __unused)
 	    thread_reap_callout_cb, NULL);
 }
 
+/*
+ * Calling this function guarantees that any thread that exited before
+ * the call is reaped when the function returns.  By 'exited' we mean
+ * a thread removed from the process linkage with thread_unlink().
+ * Practically this means that caller must lock/unlock corresponding
+ * process lock before the call, to synchronize with thread_exit().
+ */
+void
+thread_reap_barrier(void)
+{
+	struct thread *td;
+	struct task *t;
+
+	td = curthread;
+
+	/*
+	 * First do context switches to each CPU to ensure that all
+	 * PCPU pc_deadthreads are moved to zombie list.
+	 */
+	quiesce_all_cpus("", PDROP);
+
+	/*
+	 * Second, fire the task in the same thread as normal
+	 * thread_reap() is done, to serialize reaping.
+	 */
+	t = malloc(sizeof(*t), M_TEMP, M_WAITOK);
+	TASK_INIT(t, 0, thread_reap_task_cb, t);
+	taskqueue_enqueue(taskqueue_thread, t);
+	taskqueue_drain(taskqueue_thread, t);
+	free(t, M_TEMP);
+}
+
 /*
  * Allocate a thread.
  */

sys/sys/proc.h

@@ -1189,6 +1189,7 @@ int	thread_create(struct thread *td, struct rtprio *rtp,
 void	thread_exit(void) __dead2;
 void	thread_free(struct thread *td);
 void	thread_link(struct thread *td, struct proc *p);
+void	thread_reap_barrier(void);
 int	thread_single(struct proc *p, int how);
 void	thread_single_end(struct proc *p, int how);
 void	thread_stash(struct thread *td);