Re: [REVIEW][PATCH] pidns: Fix hang in zap_pid_ns_processes by sending a potentially extra wakeup

[Oleg Nesterov <oleg@redhat.com>]

Sorry for delay, vacation.

On 08/30, Eric W. Biederman wrote:
>
> --- 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 think the patch is fine, and this matches "init_pids" in
zap_pid_ns_processes().

But, Eric, if this patch was not applied yet, any chance you can
add a comment ? Just a little note about the potential zombie
leader can help to understand this code. I won't insist of course,
but this "case 2" doesn't look obvious.

Off topic. What if the first alloc_pid() succeeds and then later
copy_process() fails. In this case free_pid() is called but
PIDNS_HASH_ADDING was not cleared, we miss kern_unmount(), no?

Oleg.