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SubjectRe: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd_poll
On Thu, Jul 02, 2020 at 11:41:37AM -0500, Eric W. Biederman wrote:
> Create an independent helper thread_group_exited report return true
> when all threads have passed exit_notify in do_exit. AKA all of the
> threads are at least zombies and might be dead or completely gone.
>
> Create this helper by taking the logic out of pidfd_poll where
> it is already tested, and adding a missing READ_ONCE on
> the read of task->exit_state.
>
> I will be changing the user mode driver code to use this same logic
> to know when a user mode driver needs to be restarted.
>
> Place the new helper thread_group_exited in kernel/exit.c and
> EXPORT it so it can be used by modules.
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
> include/linux/sched/signal.h | 2 ++
> kernel/exit.c | 24 ++++++++++++++++++++++++
> kernel/fork.c | 6 +-----
> 3 files changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 0ee5e696c5d8..1bad18a1d8ba 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -674,6 +674,8 @@ static inline int thread_group_empty(struct task_struct *p)
> #define delay_group_leader(p) \
> (thread_group_leader(p) && !thread_group_empty(p))
>
> +extern bool thread_group_exited(struct pid *pid);
> +
> extern struct sighand_struct *__lock_task_sighand(struct task_struct *task,
> unsigned long *flags);
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index d3294b611df1..a7f112feb0f6 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1713,6 +1713,30 @@ COMPAT_SYSCALL_DEFINE5(waitid,
> }
> #endif
>
> +/**
> + * thread_group_exited - check that a thread group has exited
> + * @pid: tgid of thread group to be checked.
> + *
> + * Test if thread group is has exited (all threads are zombies, dead
> + * or completely gone).
> + *
> + * Return: true if the thread group has exited. false otherwise.
> + */
> +bool thread_group_exited(struct pid *pid)
> +{
> + struct task_struct *task;
> + bool exited;
> +
> + rcu_read_lock();
> + task = pid_task(pid, PIDTYPE_PID);
> + exited = !task ||
> + (READ_ONCE(task->exit_state) && thread_group_empty(task));
> + rcu_read_unlock();
> +
> + return exited;
> +}

I'm not sure why you think READ_ONCE was missing.
It's different in wait_consider_task() where READ_ONCE is needed because
of multiple checks. Here it's done once.

The rest all looks good to me. Tested with and without bpf_preload patches.
Feel free to create a frozen branch with this set.

btw I'll be offline starting tomorrow for a week.
Will catch up with threads afterwards.

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