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

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Date:	Thu, 29 Aug 2013 15:08:41 -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

Commit 751c644b95bb48aaa8825f0c66abbcc184d92051 also played a role
where we started handling multi-threaded inits but the wake-up remains
broken.

> 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.

I completely agree with your problem analysis.  All we hold in
free_pid is the pidmap_lock.  Not the task_lock which guards
ns->child_reaper nor the signhand lock which guards PF_EXITING.

I think a final patch needs an analysis why whichever wakeup scheme we
use does not have races which will result in the failure to send a
wakeup.

Using a default case and PF_EXITING test while retaing the previous
nr_hashed == 1 seems a little hacky.

Regardless thank you for all of your hard work to track this one down.

I feel silly for not considering the wakeup side before.

> 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);
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