Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks

[Paul E. McKenney]

On Thu, Jun 28, 2018 at 02:13:15PM -0700, Joel Fernandes wrote:
> On Thu, Jun 28, 2018 at 01:02:05PM -0700, Paul E. McKenney wrote:
> > > > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> [..]
> > > > > > > > > So why this function-call structure?  Well, you see, NMI handlers can
> > > > > > > > > take what appear to RCU to be normal interrupts...
> > > > > > > > >
> > > > > > > > > (And I just added that fun fact to Requirements.html.)
> > > > > > > >
> > > > > > > > Yes, I'll definitely go through all the interrupt requirements in the doc and
> > > > > > > > thanks for referring me to it.
> > > > > > >
> > > > > > > My concern may well be obsolete.  It would be good if it was!  ;-)
> > > > > >
> > > > > > I'd love to mandate that irq_enter() must be paired with irq_exit(). I
> > > > > > don't really see any rationale for it to be otherwise. If there is a
> > > > > > case, perhaps it needs to be fixed.
> > > > >
> > > > > Given that the usermode helpers now look to be common code using
> > > > > workqueues, kthreads, and calls to do_execve(), it might well be that
> > > > > the days of half-interrupts are behind us.
> > > > >
> > > > > But how to actually validate this?  My offer of adding a WARN_ON_ONCE()
> > > > > and waiting a few years still stands, but perhaps you have a better
> > > > > approach.
> > > > 
> > > > I think you should add a WARN_ON_ONCE().  Let's get the bugs fixed.
> > > 
> > > Or the obscure features identified, as the case may be.  ;-)
> > > 
> > > Either way, will do!
> > 
> > And here is a prototype patch.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit ef544593a7bcad74628fa0537badc49dce1f2d95
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Date:   Thu Jun 28 12:45:23 2018 -0700
> > 
> >     rcu: Add warning to detect half-interrupts
> >     
> >     RCU's dyntick-idle code is written to tolerate half-interrupts, that it,
> >     either an interrupt that invokes rcu_irq_enter() but never invokes the
> >     corresponding rcu_irq_exit() on the one hand, or an interrupt that never
> >     invokes rcu_irq_enter() but does invoke the "corresponding" rcu_irq_exit()
> >     on the other.  These things really did happen at one time, as evidenced
> >     by this ca-2011 LKML post:
> >     
> >     http://lkml.kernel.org/r/20111014170019.GE2428@xxxxxxxxxxxxxxxxxx
> >     
> >     The reason why RCU tolerates half-interrupts is that usermode helpers
> >     used exceptions to invoke a system call from within the kernel such that
> >     the system call did a normal return (not a return from exception) to
> >     the calling context.  This caused rcu_irq_enter() to be invoked without
> >     a matching rcu_irq_exit().  However, usermode helpers have since been
> >     rewritten to make much more housebroken use of workqueues, kernel threads,
> >     and do_execve(), and therefore should no longer produce half-interrupts.
> >     No one knows of any other source of half-interrupts, but then again,
> >     no one seems insane enough to go audit the entire kernel to verify that
> >     half-interrupts really are a relic of the past.
> >     
> >     This commit therefore adds a pair of WARN_ON_ONCE() calls that will
> >     trigger in the presence of half interrupts, which the code will continue
> >     to handle correctly.  If neither of these WARN_ON_ONCE() trigger by
> >     mid-2021, then perhaps RCU can stop handling half-interrupts, which
> >     would be a considerable simplification.
> >     
> >     Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> >     Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> >     Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> Looks good to me!
> 
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Applied, thank you!!!

							Thanx, Paul