[PATCH 2/2] pid: document the PIDNS_ADDING checks in alloc_pid() and copy_process()

[Oleg Nesterov <oleg@redhat.com>]

Both copy_process() and alloc_pid() do the same PIDNS_ADDING check.
The reasons for these checks, and the fact that both are necessary,
are not immediately obvious. Add the comments.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/fork.c | 6 +++++-
 kernel/pid.c  | 5 +++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 544fe1b43d88..7cfa8addc080 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2392,7 +2392,11 @@ __latent_entropy struct task_struct *copy_process(
 
 	rseq_fork(p, clone_flags);
 
-	/* Don't start children in a dying pid namespace */
+	/*
+	 * If zap_pid_ns_processes() was called after alloc_pid(), the new
+	 * child missed SIGKILL.  If current is not in the same namespace,
+	 * we can't rely on fatal_signal_pending() below.
+	 */
 	if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
 		retval = -ENOMEM;
 		goto bad_fork_core_free;
diff --git a/kernel/pid.c b/kernel/pid.c
index 1a0d2ac1f4a9..082a3c4a053f 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -317,6 +317,11 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 	 *
 	 * This can't be done earlier because we need to preserve other
 	 * error conditions.
+	 *
+	 * We need this even if copy_process() does the same check. If two
+	 * or more tasks from parent namespace try to inject a child into a
+	 * dead namespace, one of free_pid() calls from the copy_process()
+	 * error path may try to wakeup the possibly freed ns->child_reaper.
 	 */
 	retval = -ENOMEM;
 	if (unlikely(!(ns->pid_allocated & PIDNS_ADDING)))
-- 
2.52.0