linux-kernel - Re: WARNING: suspicious RCU usage in idtentry

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Message-ID: <20200528204839.GR2869@paulmck-ThinkPad-P72>
Date:   Thu, 28 May 2020 13:48:39 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     syzbot <syzbot+3ae5eaae0809ee311e75@...kaller.appspotmail.com>,
        Paolo Bonzini <pbonzini@...hat.com>, bp@...en8.de,
        hpa@...or.com, linux-kernel@...r.kernel.org, luto@...nel.org,
        mingo@...nel.org, syzkaller-bugs@...glegroups.com, x86@...nel.org
Subject: Re: WARNING: suspicious RCU usage in idtentry_exit

On Thu, May 28, 2020 at 10:19:02PM +0200, Thomas Gleixner wrote:
> Paul,
> 
> "Paul E. McKenney" <paulmck@...nel.org> writes:
> > On Thu, May 28, 2020 at 03:33:44PM +0200, Thomas Gleixner wrote:
> >> syzbot <syzbot+3ae5eaae0809ee311e75@...kaller.appspotmail.com> writes:
> >> Weird. I have no idea how that thing is an EQS here.
> >
> > No argument on the "Weird" part!  ;-)
> >
> > Is this a NO_HZ_FULL=y kernel?
> 
> No, it has only NO_HZ_IDLE.
> 
>   https://syzkaller.appspot.com/x/.config?x=47b0740d89299c10

OK, from the .config, another suggestion is to build the kernel
with CONFIG_RCU_EQS_DEBUG=y.  This still requires that this issue be
reproduced, but it might catch the problem earlier.

> > If so, one possibility is that the call
> > to rcu_user_exit() went missing somehow.  If not, then RCU should have
> > been watching userspace execution.
> >
> > Again, the only thing I can think of (should this prove to be
> > reproducible) is the rcu_dyntick trace event.
> 
> :)
> 
> Thanks,
> 
>         tglx

							Thanx, Paul