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

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Message-Id: <20180622211600.GX3593@linux.vnet.ibm.com>
Date:   Fri, 22 Jun 2018 14:16:00 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Byungchul Park <max.byungchul.park@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        jiangshanlai@...il.com, josh@...htriplett.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com, luto@...nel.org
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks

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@...ux.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