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    SubjectRe: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks
    On Fri, Jun 22, 2018 at 03:03:35PM -0700, Andy Lutomirski wrote:
    > On Fri, Jun 22, 2018 at 2:14 PM Paul E. McKenney
    > <paulmck@linux.vnet.ibm.com> wrote:
    > >
    > > 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" <paulmck@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.
    >
    > 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!

    Thanx, Paul

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