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

[Eric W. Biederman]

Frederic Weisbecker <frederic@xxxxxxxxxx> writes:

> RCU Tasks and PID-namespace unshare can interact in do_exit() in a
> complicated circular dependency:
>
> 1) TASK A calls unshare(CLONE_NEWPID), this creates a new PID namespace
>    that every subsequent child of TASK A will belong to. But TASK A
>    doesn't itself belong to that new PID namespace.
>
> 2) TASK A forks() and creates TASK B. TASK A stays attached to its PID
>    namespace (let's say PID_NS1) and TASK B is the first task belonging
>    to the new PID namespace created by unshare()  (let's call it PID_NS2).
>
> 3) Since TASK B is the first task attached to PID_NS2, it becomes the
>    PID_NS2 child reaper.
>
> 4) TASK A forks() again and creates TASK C which get attached to PID_NS2.
>    Note how TASK C has TASK A as a parent (belonging to PID_NS1) but has
>    TASK B (belonging to PID_NS2) as a pid_namespace child_reaper.
>
> 5) TASK B exits and since it is the child reaper for PID_NS2, it has to
>    kill all other tasks attached to PID_NS2, and wait for all of them to
>    die before getting reaped itself (zap_pid_ns_process()).
>
> 6) TASK A calls synchronize_rcu_tasks() which leads to
>    synchronize_srcu(&tasks_rcu_exit_srcu).
>
> 7) TASK B is waiting for TASK C to get reaped. But TASK B is under a
>    tasks_rcu_exit_srcu SRCU critical section (exit_notify() is between
>    exit_tasks_rcu_start() and exit_tasks_rcu_finish()), blocking TASK A.
>
> 8) TASK C exits and since TASK A is its parent, it waits for it to reap
>    TASK C, but it can't because TASK A waits for TASK B that waits for
>    TASK C.
>
> Pid_namespace semantics can hardly be changed at this point. But the
> coverage of tasks_rcu_exit_srcu can be reduced instead.
>
> The current task is assumed not to be concurrently reapable at this
> stage of exit_notify() and therefore tasks_rcu_exit_srcu can be
> temporarily relaxed without breaking its constraints, providing a way
> out of the deadlock scenario.
>
> Fixes: 3f95aa81d265 ("rcu: Make TASKS_RCU handle tasks that are almost done exiting")
> Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Suggested-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Eric W . Biederman <ebiederm@xxxxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  include/linux/rcupdate.h |  2 ++
>  kernel/pid_namespace.c   | 17 +++++++++++++++++
>  kernel/rcu/tasks.h       | 14 ++++++++++++--
>  3 files changed, 31 insertions(+), 2 deletions(-)

> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index f4f8cb0435b4..fc21c5d5fd5d 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -244,7 +244,24 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>  		set_current_state(TASK_INTERRUPTIBLE);
>  		if (pid_ns->pid_allocated == init_pids)
>  			break;
> +		/*
> +		 * Release tasks_rcu_exit_srcu to avoid following deadlock:
> +		 *
> +		 * 1) TASK A unshare(CLONE_NEWPID)
> +		 * 2) TASK A fork() twice -> TASK B (child reaper for new ns)
> +		 *    and TASK C
> +		 * 3) TASK B exits, kills TASK C, waits for TASK A to reap it
> +		 * 4) TASK A calls synchronize_rcu_tasks()
> +		 *                   -> synchronize_srcu(tasks_rcu_exit_srcu)
> +		 * 5) *DEADLOCK*
> +		 *
> +		 * It is considered safe to release tasks_rcu_exit_srcu here
> +		 * because we assume the current task can not be concurrently
> +		 * reaped at this point.
> +		 */
> +		exit_tasks_rcu_stop();
>  		schedule();
> +		exit_tasks_rcu_start();
>  	}
>  	__set_current_state(TASK_RUNNING);

Two questions.

1) Is there any chance you need the exit_task_rcu_stop() and
   exit_tasks_rcu_start() around schedule in the part of this code that
   calls kernel_wait4.

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,
   and instead have wait_consider_task simply not allow the 
   init process of the pid namespace to be reaped.

   Am I right in thinking that such a change were to be made it would
   make remove the deadlock without having to have any special code?

   It is just tricky enough to do that I don't want to discourage your
   simpler change but this looks like a case that makes the pain of
   changing zap_pid_ns_processes worthwhile in the practice.

Eric