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
