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Date:   Thu, 28 Jun 2018 14:13:15 -0700
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        max.byungchul.park@gmail.com,
        Byungchul Park <byungchul.park@lge.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@lge.com
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks
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On Thu, Jun 28, 2018 at 01:02:05PM -0700, Paul E. McKenney wrote:
> > > <paulmck@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 <paulmck@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.GE2428@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 <rostedt@goodmis.org>
>     Reported-by: Joel Fernandes <joel@joelfernandes.org>
>     Reported-by: Andy Lutomirski <luto@kernel.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Looks good to me!

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

- Joel