linux-kernel - Re: [PATCH] fix problem with reschenduling in rest

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Date:	Thu, 24 Jun 2010 15:11:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ilya Loginov <isloginov@...il.com>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] fix problem with reschenduling in rest_init
 (2.6.35-rc3)

On Thu, 2010-06-24 at 00:11 +0400, Ilya Loginov wrote:
> 	There is a bug in rest_init function. The problem is that kernel_init
> thread starts before initialization of kthreadd_task when
> CONFIG_PREEMPT_VOLUNTARY is enabled.
> 
> kernel_init thread do wake_up_process(kthreadd_task) and I have kernel Oops in
> try_to_wake_up when I try to get p->state.
> 
> 	I found this problem on 2.6.34 on FPGA. It is very slow, and
> find_task_by_pid is done after reschenduling. I have no this problem on
> 2.6.35-rc3 because kernel code is moved. But if I write simple loop like
> 	volatile int tmp;
> 	for(i = 0; i < preset_lpj; i++)
> 		tmp++;
> right after kernel_thread(kernel_init,.... , I have this problem on
> 2.6.35-rc3 too.
> 
> 	I understand that real problem is reschenduling in init code, but
> I have no ability to fix it.

Ooh, interesting problem, so kernel_init() will indeed spawn all kinds
of kernel threads, and doing so before we create the kthreadd will
result in the oops you observed.

However I suspect the ordering is like it is because we want init to
have pid 1, if we were to re-order like you suggest kthreadd will end up
with pid 1 and init with pid 2.

Humm, how to fix this...

> Signed-off-by: Ilya Loginov <isloginov@...il.com>
> ---
> diff --git a/init/main.c b/init/main.c
> index 3bdb152..9febd69 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -428,12 +428,12 @@ static noinline void __init_refok rest_init(void)
>  	int pid;
>  
>  	rcu_scheduler_starting();
> -	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();
> +	kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
>  	unlock_kernel();
>  
>  	/*

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