Re: [PATCH v2 1/2] Add polling support to pidfd

On 04/30, Joel Fernandes (Google) wrote:

+static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts) +{

• struct task_struct *task;
• struct pid *pid = file->private_data;
• int poll_flags = 0;
• poll_wait(file, &pid->wait_pidfd, pts);
• rcu_read_lock();
• task = pid_task(pid, PIDTYPE_PID);
• WARN_ON_ONCE(task && !thread_group_leader(task));

^^^^^^^^^^^^^^^^^^^^^^^^^^

Ah, this is not right, we can race with de_thread() which changes the leader, in particular it does leader->exit_signal = -1 to indicate that this thread is no longer a group leader, but pid_task() can return the old leader.

We are going to check thread_group_empty() below, it won't be true in this case, so this race should not make any harm.

Just remove this WARN_ON(). We can't use has_group_leader_pid(), it can return false if pid_task() returns the new leader.

Otherwise I see no problems.

Oleg.