Re: [PATCH 3/3] rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()

[Paul E. McKenney]

On Tue, Dec 06, 2022 at 05:49:28PM +0100, Oleg Nesterov wrote:
> On 11/30, Eric W. Biederman wrote:
> >
> > 2) I keep thinking zap_pid_ns_processes() should be changed so that
> >    after it sends SIGKILL to all of the relevant processes to not wait,
> 
> At least I think it should not wait for the tasks injected into this ns.
> 
> Because this looks like a kernel bug even if we forget about this deadlock.
> 
> Say we create a task P using clone(CLONE_NEWPID), then inject a task T into
> P's pid-namespace via setns/fork. This make the process P "unkillable", it
> will hang in zap_pid_ns_processes() "forever" until T->parent reaps a zombie
> task T killed by P.

Given that this is not the first time that Tasks Trace RCU has been
involved in a deadlock involving this exit path, I could certainly get
behind this approach.  ;-)

							Thanx, Paul