linux-kernel - Re: [PATCH] Make sure to wake reaper

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Date:	Thu, 29 Aug 2013 17:39:14 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Serge Hallyn <serge.hallyn@...ntu.com>
Cc:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] Make sure to wake reaper

Serge Hallyn <serge.hallyn@...ntu.com> writes:

> Since commit af4b8a83add95ef40716401395b44a1b579965f4 it's been
> possible to get into a situation where a pidns reaper is
> <defunct>, reparented to host pid 1, but never reaped.  How to
> reproduce this is documented at
>
> https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1168526
> (and see
> https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1168526/comments/13)
> In short, run repeated starts of a container whose init is
>
> Process.exit(0);
>
> sysrq-t when such a task is playing zombie shows:
>
> [  131.132978] init            x ffff88011fc14580     0  2084   2039 0x00000000
> [  131.132978]  ffff880116e89ea8 0000000000000002 ffff880116e89fd8 0000000000014580
> [  131.132978]  ffff880116e89fd8 0000000000014580 ffff8801172a0000 ffff8801172a0000
> [  131.132978]  ffff8801172a0630 ffff88011729fff0 ffff880116e14650 ffff88011729fff0
> [  131.132978] Call Trace:
> [  131.132978]  [<ffffffff816f6159>] schedule+0x29/0x70
> [  131.132978]  [<ffffffff81064591>] do_exit+0x6e1/0xa40
> [  131.132978]  [<ffffffff81071eae>] ? signal_wake_up_state+0x1e/0x30
> [  131.132978]  [<ffffffff8106496f>] do_group_exit+0x3f/0xa0
> [  131.132978]  [<ffffffff810649e4>] SyS_exit_group+0x14/0x20
> [  131.132978]  [<ffffffff8170102f>] tracesys+0xe1/0xe6
>
> Further debugging showed that every time this happened, zap_pid_ns_processes()
> started with nr_hashed being 3, while we were expecting it to drop to 2.
> Any time it didn't happen, nr_hashed was 1 or 2.  So the reaper was
> waiting for nr_hashed to become 2, but free_pid() only wakes the reaper
> if nr_hashed hits 1.  This patch makes free_pid() wake the reaper any
> time the reaper is PF_EXITING, to force it to re-test the
> pidns->nr_hashed = init_pids test.  Note that this is more like what
> __unhash_process() used to do before
> af4b8a83add95ef40716401395b44a1b579965f4.
>
> Signed-off-by: Serge Hallyn <serge.hallyn@...ntu.com>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  kernel/pid.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 0db3e79..6b312c4 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -274,6 +274,10 @@ void free_pid(struct pid *pid)
>  		case 0:
>  			schedule_work(&ns->proc_work);
>  			break;
> +		default:
> +			if (ns->child_reaper->flags & PF_EXITING)
> +				wake_up_process(ns->child_reaper);
> +			break;
>  		}
>  	}
>  	spin_unlock_irqrestore(&pidmap_lock, flags);

qSo I think the change that we actually want is just to send a wake-up
when we have two pids in the pid namespace as well as one pid.

- That can send one extraneous wake-up but that is relatively harmless.
- We can detect the condition race free.
- With only two pids remaining we are guaranteed that which ever task is
  the child_reaper will persist through zap_pid_ns_processes.

  There are 3 cases.
  init-tgleader other -- Single threaded init so of course we won't free the task
  init-tgleader-dead init-thread -- The last living init thread will call zap_pid_ns_processes.
  init-tgleader init-thread -- An init with two living threads child_reaper must be the init thread group leader

Which means at the cost of an extra wake-up we are guaranteed not to
have races.

Serge does that look good to you?

Eric



diff --git a/kernel/pid.c b/kernel/pid.c
index 17755ae..ab75add 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -265,6 +265,7 @@ void free_pid(struct pid *pid)
                struct pid_namespace *ns = upid->ns;
                hlist_del_rcu(&upid->pid_chain);
                switch(--ns->nr_hashed) {
+               case 2:
                case 1:
                        /* When all that is left in the pid namespace
                         * is the reaper wake up the reaper.  The reaper
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