Re: [PATCH] init: Fix race between init and kthreadd -v3

Randy Dunlap
2010-06-28T16:26:12Z

On Mon, 28 Jun 2010 16:51:01 +0200 Peter Zijlstra wrote:

> On Mon, 2010-06-28 at 16:19 +0200, Ingo Molnar wrote:
>
> > I think you may be using a mutex as a completion in essence. Why not use
> > completions instead?
>
> Totally forgot about those things.. Yes they fit perfectly.
>
> ---
> Subject: init: Fix race between init and kthreadd -v2
>
> Ilya reported that on a very slow machine he could reliably reproduce a
> race between forking init and kthreadd. We first fork init so that it
> obtains pid-1, however since the scheduler is already fully running at
> this point it can preempt and run the init thread before we spawn and
> set kthreadd_task.
>
> The init thread can then attempt spawning kthreads without kthreadd
> being present which results in an OOPS.
>
> Reported-by: Ilya Loginov <isloginov@gmail.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> init/main.c | 12 ++++++++++++
> 1 files changed, 12 insertions(+), 0 deletions(-)
>
> diff --git a/init/main.c b/init/main.c
> index e2a2bf3..2280f63 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -420,18 +420,26 @@ static void __init setup_command_line(char *command_line)
> * gcc-3.4 accidentally inlines this function, so use noinline.
> */
>
> +static __initdata DECLARE_COMPLETION(kthreadd_done);
> +
> static noinline void __init_refok rest_init(void)
> __releases(kernel_lock)
> {
> int pid;
>
> rcu_scheduler_starting();
> + /*
> + * We need to spawn init first so that it obtains pid-1, however

Would be better to say "pid 1" or "pid=1".
"pid-1" seems odd to me.

> + * the init task will end up wanting to create kthreads, which, if
> + * we schedule it before we create kthreadd, will OOPS.
> + */
> kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
> numa_default_policy();
> pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
> rcu_read_lock();
> kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
> rcu_read_unlock();
> + complete(&kthreadd_done);
> unlock_kernel();
>
> /*
> @@ -847,6 +855,10 @@ static noinline int init_post(void)
>
> static int __init kernel_init(void * unused)
> {
> + /*
> + * Wait until kthreadd is all set-up.
> + */
> + wait_for_completion(&kthreadd_done);
> lock_kernel();
>
> /*
>
> --

---
~Randy
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