Re: WARNING: suspicious RCU usage in idtentry_exit

[Paul E. McKenney]

On Fri, May 29, 2020 at 08:20:12AM +0200, Dmitry Vyukov wrote:
> On Thu, May 28, 2020 at 10:48 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Thu, May 28, 2020 at 10:19:02PM +0200, Thomas Gleixner wrote:
> > > Paul,
> > >
> > > "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:
> > > > On Thu, May 28, 2020 at 03:33:44PM +0200, Thomas Gleixner wrote:
> > > >> syzbot <syzbot+3ae5eaae0809ee311e75@xxxxxxxxxxxxxxxxxxxxxxxxx> 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).

It is an extra WARN_ON_ONCE() with a simple comparison, but on almost
every kernel entry/exit path.

So not something you want in production, but much lighter weight than
any of the tools you listed above.

Full disclosure:  It usually fires for new architectures or for new
timer hardware/drivers.  Which might allow you to enable it selectively.

							Thanx, Paul

> > > > 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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