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

[Joel Fernandes]

On Sat, Jun 23, 2018 at 10:56:34AM -0700, Paul E. McKenney wrote:
> On Sat, Jun 23, 2018 at 08:48:39AM -0700, Joel Fernandes wrote:
> > On Fri, Jun 22, 2018 at 02:16:00PM -0700, Paul E. McKenney 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@xxxxxxxxxxxxxxxxxx> 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.
> > 
> > Hi Paul, I am Ok with adding a warning for a couple of releases if you and
> > others are Ok with it, how about something like this? Feel free to use the
> > diff as a starting point or a different approach if you/others prefer
> > something else. Thanks.
> 
> A few years rather than a few releases, but yes.  ;-)
> The checks would need to go just before the "crowbar" stores.  

Ok. I guess that would work too.

> I will put something together after Byungchul's patches in this area have
> had time to burn in for a few days.

Sounds great, thanks!

 - Joel