Messages in this thread Patch in this message | | | From | (Eric W. Biederman) | Date | Thu, 29 Aug 2013 17:39:14 -0700 | Subject | Re: [PATCH] Make sure to wake reaper |
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Serge Hallyn <serge.hallyn@ubuntu.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@ubuntu.com> > Cc: "Eric W. Biederman" <ebiederm@xmission.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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