linux-kernel - Re: WARNING: suspicious RCU usage (3)

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Message-Id: <20171129205059.GI3624@linux.vnet.ibm.com>
Date:   Wed, 29 Nov 2017 12:50:59 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        syzbot 
        <bot+73a7bec1bc0f4fc0512a246334081f8c671762a8@...kaller.appspotmail.com>,
        Christoph Lameter <cl@...ux.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        syzkaller-bugs@...glegroups.com,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: WARNING: suspicious RCU usage (3)

On Wed, Nov 29, 2017 at 07:25:32AM +0100, Dmitry Vyukov wrote:
> On Tue, Nov 28, 2017 at 11:30 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Tue, Nov 28, 2017 at 01:30:26PM -0800, Andrew Morton wrote:
> >> On Tue, 28 Nov 2017 12:45:01 -0800 syzbot <bot+73a7bec1bc0f4fc0512a246334081f8c671762a8@...kaller.appspotmail.com> wrote:
> >>
> >> > Hello,
> >> >
> >> > syzkaller hit the following crash on
> >> > b0a84f19a5161418d4360cd57603e94ed489915e
> >> > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
> >> > compiler: gcc (GCC) 7.1.1 20170620
> >> > .config is attached
> >> > Raw console output is attached.
> >> >
> >> > Unfortunately, I don't have any reproducer for this bug yet.
> >> >
> >> > WARNING: suspicious RCU usage
> >>
> >> There's a bunch of other info which lockdep_rcu_suspicious() should
> >> have printed out, but this trace doesn't have any of it.  I wonder why.
> >
> > Yes, there should be more info printed, no idea why it would go missing.
> 
> I think that's because while reporting "suspicious RCU usage" kernel hit:
> 
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000074
> 
> and the rest of the report is actually about the NULL deref.
> 
> syzkaller hits too many crashes at the same time. And it's a problem
> for us. We get reports with corrupted/intermixed titles,
> corrupted/intermixed bodies, reports with same titles but about
> different bugs, etc.

Got it, thank you!

							Thanx, Paul