linux-kernel - Re: [lkp] [torture] 945fa9c631b: BUG: unable to handle kernel NULL pointer dereference at (null)

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Message-ID: <20150714114848.238118dd@gandalf.local.home>
Date:	Tue, 14 Jul 2015 11:48:48 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Huang Ying <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, LKP ML <lkp@...org>
Subject: Re: [lkp] [torture] 945fa9c631b: BUG: unable to handle kernel NULL
 pointer dereference at           (null)

On Tue, 14 Jul 2015 08:38:29 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Tue, Jul 14, 2015 at 09:04:08PM +0800, Huang Ying wrote:
> > FYI, we noticed the below changes on
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > commit 945fa9c631b04febe295a3a2a00c7e4a3cfb97db ("torture: Dump ftrace buffer when the RCU grace period stalls")
> > 
> > We think the commit may reveal a existing bug.
> 
> Hmmm...  Is this perhaps a very slow boot due to diagnostics?  If so,
> might the ftrace buffer not yet be initialized?  CCing Steven Rostedt
> for his thoughts.

Is this reproducible?

Does the following patch fix it for you?

-- Steve

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6e79408674aa..4d68dc121687 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6968,6 +6968,9 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 	unsigned long flags;
 	int cnt = 0, cpu;
 
+	if (tracing_disabled)
+		return;
+
 	/* Only allow one dump user at a time. */
 	if (atomic_inc_return(&dump_running) != 1) {
 		atomic_dec(&dump_running);
--
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