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:
> > > > <paul...@linux.vnet.ibm.com> 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 <paul...@linux.vnet.ibm.com>
> > 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.ge2...@linux.vnet.ibm.com
> >
> > 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 <rost...@goodmis.org>
> > Reported-by: Joel Fernandes <j...@joelfernandes.org>
> > Reported-by: Andy Lutomirski <l...@kernel.org>
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> Looks good to me!
>
> Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org>
Applied, thank you!!!
Thanx, Paul
