On Fri, Jun 22, 2018 at 05:00:42PM -0400, Steven Rostedt wrote:
> On Fri, 22 Jun 2018 13:58:13 -0700
> "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
>
> > Something like this:
> >
> > IRQ entered
> >
> > And never exited. Ever. I actually saw this in 2011.
>
> I still believe this was actually a bug. And perhaps you made the RCU
> code robust enough to handle this bug ;-)
Welcome to my world!
But I recall it being used in several places, so if it was a bug, it
was an intentional bug. Probably the worst kind.
Sort of like nested NMIs and interrupts within NMI handlers. ;-)
> > Or something like this:
> >
> > IRQ exited
> >
> > Without a corresponding IRQ enter.
> >
> > The current code handles both of these situations, at least assuming
> > that the interrupt entry/exit happens during a non-idle period.
> >
> > > > 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.
Thanx, Paul
