Re: initcall dependency problem (ns vs. threads)

[Andrew Morton]

On Mon, 1 Aug 2011 22:01:51 +0400 Vasiliy Kulikov <segoon@xxxxxxxxxxxx> wrote:

> 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
>     }

There's not really enough detail here for me to suggest a fix without
actually doing some work.  Which ipc initialization function is being
called to late?  Which thread is using which data structures before
which initialization function has been run?

Are we talking about init_ipc_ns.ids[] here?  If so, did you try
initializing the three rwsems at compile-time?

That's rather a nasty hack though.  It'd be better to run the mystery
init function before starting the threads.
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