Re: [PATCH v2 2/2] proc: Ensure we see the exit of each process tid exactly

Oleg Nesterov
2020-04-24T17:39:37Z

On 04/23, Eric W. Biederman wrote:
>
> When the thread group leader changes during exec and the old leaders
> thread is reaped proc_flush_pid

This is off-topic, but let me mention this before I forget...

Note that proc_flush_pid() does nothing if CONFIG_PROC_FS=n, this mean
that in this case release_task() leaks thread_pid.

> +void exchange_tids(struct task_struct *ntask, struct task_struct *otask)
> +{
> + /* pid_links[PIDTYPE_PID].next is always NULL */
> + struct pid *npid = READ_ONCE(ntask->thread_pid);
> + struct pid *opid = READ_ONCE(otask->thread_pid);
> +
> + rcu_assign_pointer(opid->tasks[PIDTYPE_PID].first, &ntask->pid_links[PIDTYPE_PID]);
> + rcu_assign_pointer(npid->tasks[PIDTYPE_PID].first, &otask->pid_links[PIDTYPE_PID]);
> + rcu_assign_pointer(ntask->thread_pid, opid);
> + rcu_assign_pointer(otask->thread_pid, npid);
> + WRITE_ONCE(ntask->pid_links[PIDTYPE_PID].pprev, &opid->tasks[PIDTYPE_PID].first);
> + WRITE_ONCE(otask->pid_links[PIDTYPE_PID].pprev, &npid->tasks[PIDTYPE_PID].first);
> + WRITE_ONCE(ntask->pid, pid_nr(opid));
> + WRITE_ONCE(otask->pid, pid_nr(npid));
> +}

Oh, at first glance this breaks posix-cpu-timers.c:lookup_task(), the last
user of has_group_leader_pid().

I think that we should change lookup_task() to return "struct *pid", this
should simplify the code... Note that none of its callers needs task_struct.

And, instead of thread_group_leader/has_group_leader_pid checks we should
use pid_has_task(TGID).

After that, this patch should kill has_group_leader_pid().

What do you think?

Oleg.