linux-kernel - Re: [sched] BUG: unable to handle kernel NULL pointer dereference at 0000000000000040

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Date:	Sat, 11 Oct 2014 06:23:20 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org
Subject: Re: [sched] BUG: unable to handle kernel NULL pointer dereference at
 0000000000000040

Yes, thanks.

How come it is sent to whole list. Please ignore this. Sorry.

On Sat, Oct 11, 2014 at 01:00:48AM -0500, Chuck Ebbert wrote:
> On Sat, 11 Oct 2014 13:15:30 +0800
> Fengguang Wu <fengguang.wu@...el.com> wrote:
> 
> > FYI, we noticed the below changes on commit
> > 
> > 445d95d7c384741d133251a9adac935866591c92 ("sched: Remove update_rq_runnable_avg")
> > 
> > [   67.303839] BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
> > [   67.304014] IP: [<ffffffff810b1d52>] print_cfs_rq+0x4a3/0xa96
> 
> Well that one's pretty obvious:
> 
> 
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -68,14 +68,6 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
>  #define PN(F) \
>  	SEQ_printf(m, "  .%-30s: %lld.%06ld\n", #F, SPLIT_NS((long long)F))
>  
> -	if (!se) {
> -		struct sched_avg *avg = &cpu_rq(cpu)->avg;
> -		P(avg->runnable_avg_sum);
> -		P(avg->runnable_avg_period);
> -		return;
> -	}
> -
> -
>  	PN(se->exec_start);
>  	PN(se->vruntime);
>  	PN(se->sum_exec_runtime);
> 
> 
> You can remove the P() calls from that if statement, but you can't
> remove the whole thing because you will try to dereference a NULL se
> immediately afterward if you do.
> --
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