Re: [PATCH 2/4] pids: init_struct_pid.tasks should never see the swapper process

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Oleg Nesterov (oleg@redhat.com):
> "statically initialize struct pid for swapper" commit 820e45db says:
> 
> 	Statically initialize a struct pid for the swapper process (pid_t == 0)
> 	and attach it to init_task.  This is needed so task_pid(), task_pgrp()
> 	and task_session() interfaces work on the swapper process also.
> 
> OK, but:
> 
> 	- it doesn't make sense to add init_task.pids[].node into
> 	  init_struct_pid.tasks[], and in fact this just wrong.
> 
> 	  idle threads are special, they shouldn't be visible on any
> 	  global list. In particular do_each_pid_task(init_struct_pid)
> 	  shouldn't see swapper.
> 
> 	  This is the actual reason why kill(0, SIGKILL) from /sbin/init
> 	  (which starts with 0,0 special pids) crashes the kernel. The
> 	  signal sent to pgid/sid == 0 must never see idle threads, even
> 	  if the previous patch fixed the crash itself.
> 
> 	- we have other idle threads running on the non-boot CPUs, see
> 	  the next patch.
> Change INIT_STRUCT_PID/INIT_PID_LINK to create the empty/unhashed
> hlist_head/hlist_node. Like any other idle thread swapper can never exit,
> so detach_pid()->__hlist_del() is not possible, but we could change
> INIT_PID_LINK() to set pprev = &next if needed.
> 
> All we need is the valid swapper->pids[].pid == &init_struct_pid.
> 
> Reported-by: Mathias Krause <mathias.krause@secunet.com>

Crimey, trying to find some way this could get dereferenced, finding
myself impressed with the likes of set_ftrace_swapper().

Anyway, not finding anything, so

> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Serge E. Hallyn <serue@us.ibm.com>

thanks,
-serge