[PATCH] zap_pid_ns_processes: don't send SIGKILL to sub-threads

[Oleg Nesterov <oleg@redhat.com>]

The comment above the idr_for_each_entry_continue() loop tries to explain
why we have to signal each thread in the namespace, but it is outdated.
This code no longer uses kill_proc_info(), we have a target task so we can
check thread_group_leader() and avoid the unnecessary group_send_sig_info.
Better yet, we can change pid_task() to use PIDTYPE_TGID rather than _PID,
this way it returns NULL if this pid is not a group-leader pid.

Also, change this code to check SIGNAL_GROUP_EXIT, the exiting process /
thread doesn't necessarily has a pending SIGKILL. Either way these checks
are racy without siglock, so the patch uses data_race() to shut up KCSAN.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/pid_namespace.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 25f3cf679b35..0f9bd67c9e75 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -191,21 +191,14 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	 * The last thread in the cgroup-init thread group is terminating.
 	 * Find remaining pid_ts in the namespace, signal and wait for them
 	 * to exit.
-	 *
-	 * Note:  This signals each threads in the namespace - even those that
-	 * 	  belong to the same thread group, To avoid this, we would have
-	 * 	  to walk the entire tasklist looking a processes in this
-	 * 	  namespace, but that could be unnecessarily expensive if the
-	 * 	  pid namespace has just a few processes. Or we need to
-	 * 	  maintain a tasklist for each pid namespace.
-	 *
 	 */
 	rcu_read_lock();
 	read_lock(&tasklist_lock);
 	nr = 2;
 	idr_for_each_entry_continue(&pid_ns->idr, pid, nr) {
-		task = pid_task(pid, PIDTYPE_PID);
-		if (task && !__fatal_signal_pending(task))
+		task = pid_task(pid, PIDTYPE_TGID);
+		/* reading signal->flags is racy without sighand->siglock */
+		if (task && !(data_race(task->signal->flags) & SIGNAL_GROUP_EXIT))
 			group_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_MAX);
 	}
 	read_unlock(&tasklist_lock);
-- 
2.25.1.362.g51ebf55