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

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Message-ID: <20180622170042.4adfbe21@gandalf.local.home>
Date:   Fri, 22 Jun 2018 17:00:42 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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 ;-)

> 
> 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.

-- Steve