initcall dependency problem (ns vs. threads)

Vasiliy Kulikov
2011-08-01T18:02:05Z

Hi,

There were reported problems with recent shm changes, by Manuel
Lauss (on MIPS), Richard Weinberger (on UML), and Marc Zyngier (on ARM).

https://lkml.org/lkml/2011/8/1/149
https://lkml.org/lkml/2011/8/1/162
https://lkml.org/lkml/2011/8/1/210

The problem became visible on this patch:

commit 5774ed014f02120db9a6945a1ecebeb97c2acccb
Author: Vasiliy Kulikov <segoon@openwall.com>
Date: Fri Jul 29 03:55:31 2011 +0400

shm: handle separate PID namespaces case

It started to use &shm_ids(ns).rw_mutex, which is not initialized yet.
Init IPC namespace is initialized as initcall() and some threads are
created as early_initcall().

I threat it is a dependency bug in the core kernel - kernel threads
should be able to use any namespace information, but currently there is
a race between namespace initialization code (which is initcall) and
kernel threads (which are early_initcall).

I don't feel enough experienced in init code dependencies, so I report
it to you.

static int __init kernel_init(void * unused)
{
...
do_pre_smp_initcalls(); << threads start here
...
do_basic_setup();

static void __init do_basic_setup(void)
{
cpuset_init_smp();
usermodehelper_init();
init_tmpfs();
driver_init();
init_irq_proc();
do_ctors();
do_initcalls(); << namespace init here
}

Thanks,

--
Vasiliy