Re: [PATCH] Make sure to wake reaper

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
> Serge Hallyn <serge.hallyn@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > ---
> >  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.

Would more than one extraneous wake-up be more harmful?

> - 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.

My problem is I don't really understand the assumptions behind nr_hashed.
I *thought* it was simply >1 if the init was threaded - but are threads
in init limited to 2?  Or am I totally wrong about what the 2 means?

If init *is* threaded, and the pid_ns->child_reaper exits but the other
thread is still alive, then find_new_reaper should set pid_ns->child_reaper
to the not-PF_EXITING task using

    509         while_each_thread(father, thread) {
    510                 if (thread->flags & PF_EXITING)
    511                         continue;
    512                 if (unlikely(pid_ns->child_reaper == father))
    513                         pid_ns->child_reaper = thread;
    514                 return thread;
    515         }

right?

Which seems to suggest that checking for pid_ns->child_reaper->flags &
PF_EXITING should always give us the right answer in free_pid().

>   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.

right,

>   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?

I may just need to spend a few hours going back over the old commits
and related email threads pertaining to multi-threaded inits.  I now
regret not having paid enough attention at the time :)

> 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

I considered this, but I wasn't quite sure...  I have two concersn about
the 2.  First, why can't it be 3 (3 PF_EXITING init-threads).

Second, what if nr_hashed is 2 but init in fact wasn't exiting?  Oh, that's
the one you're saying isn't an issue, just a spurious extra wakeup?  Right.

-serge
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