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

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Date:	Sat, 11 Oct 2014 01:00:48 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Yuyang Du <yuyang.du@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org
Subject: Re: [sched] BUG: unable to handle kernel NULL pointer dereference
 at 0000000000000040

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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