initcall dependency problem (ns vs. threads)

[Vasiliy Kulikov]

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@xxxxxxxxxxxx>
    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
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