Reapply "pidfd_poll: report POLLHUP when pid_task() == NULL"

This reverts commit d3ec16ca12.

Reason of revert: Fix has been applied to lmkd to handle EPOLLHUP - aosp/3268491

Change-Id: I24e17e778ff013adcbf1a6ad43470adc8d9f19cb
Signed-off-by: Terence Tritton <ttritton@google.com>
Bug: 352286227

fs/pidfs.c

@@ -89,23 +89,6 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
 }
 #endif
 
-
-static bool pidfd_task_exited(struct pid *pid, bool thread)
-{
-       struct task_struct *task;
-       bool exited;
-
-       rcu_read_lock();
-       task = pid_task(pid, PIDTYPE_PID);
-       exited = !task ||
-               (READ_ONCE(task->exit_state) && (thread || thread_group_empty(task)));
-       rcu_read_unlock();
-
-       return exited;
-}
-
-
-
 /*
  * Poll support for process exit notification.
  */
@@ -113,6 +96,7 @@ static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 {
 	struct pid *pid = pidfd_pid(file);
 	bool thread = file->f_flags & PIDFD_THREAD;
+	struct task_struct *task;
 	__poll_t poll_flags = 0;
 
 	poll_wait(file, &pid->wait_pidfd, pts);
@@ -120,8 +104,12 @@ static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 	 * Depending on PIDFD_THREAD, inform pollers when the thread
 	 * or the whole thread-group exits.
 	 */
-        if(pidfd_task_exited(pid, thread))
-            poll_flags = EPOLLIN | EPOLLRDNORM;
+	guard(rcu)();
+	task = pid_task(pid, PIDTYPE_PID);
+	if (!task)
+		poll_flags = EPOLLIN | EPOLLRDNORM | EPOLLHUP;
+	else if (task->exit_state && (thread || thread_group_empty(task)))
+		poll_flags = EPOLLIN | EPOLLRDNORM;
 
 	return poll_flags;
 }

kernel/pid.c

@@ -355,6 +355,11 @@ static void __change_pid(struct task_struct *task, enum pid_type type,
 	hlist_del_rcu(&task->pid_links[type]);
 	*pid_ptr = new;
 
+	if (type == PIDTYPE_PID) {
+		WARN_ON_ONCE(pid_has_task(pid, PIDTYPE_PID));
+		wake_up_all(&pid->wait_pidfd);
+	}
+
 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 		if (pid_has_task(pid, tmp))
 			return;