Re: WARNING: suspicious RCU usage in idtentry_exit

Dmitry Vyukov
2020-05-29T06:20:27Z

On Thu, May 28, 2020 at 10:48 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Thu, May 28, 2020 at 10:19:02PM +0200, Thomas Gleixner wrote:
> > Paul,
> >
> > "Paul E. McKenney" <paulmck@kernel.org> writes:
> > > On Thu, May 28, 2020 at 03:33:44PM +0200, Thomas Gleixner wrote:
> > >> syzbot <syzbot+3ae5eaae0809ee311e75@syzkaller.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.

How much does it slow down execution? If we enable it on syzbot, it
will affect all fuzzing done by syzbot always.
It can tolerate significant slowdown and it's far from a production
kernel (it enables KASAN, KCOV, LOCKDEP and more). But I am still
asking because some debugging features are built without performance
in mind at all (like let's just drop a global lock in every
kmalloc/free, which may be too much even for a standard debug build).

> > > 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
>
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