linux-kernel - Re: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context

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Message-ID: <20120904224839.GA11413@jtriplet-mobl1>
Date:	Tue, 4 Sep 2012 15:48:39 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used
 from interrupt context

On Tue, Sep 04, 2012 at 03:33:50PM -0700, Paul E. McKenney wrote:
> On Fri, Aug 31, 2012 at 11:00:52AM -0700, Josh Triplett wrote:
> > On Thu, Aug 30, 2012 at 11:56:17AM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > > 
> > > There is a need to use RCU from interrupt context, but either before
> > > rcu_irq_enter() is called or after rcu_irq_exit() is called.  If the
> > > interrupt occurs from idle, then lockdep-RCU will complain about such
> > > uses, as they appear to be illegal uses of RCU from the idle loop.
> > > In other environments, RCU_NONIDLE() could be used to properly protect
> > > the use of RCU, but RCU_NONIDLE() currently cannot be invoked except
> > > from process context.
> > > 
> > > This commit therefore modifies RCU_NONIDLE() to permit its use more
> > > globally.
> > > 
> > > Reported-by: Steven Rostedt <rostedt@...dmis.org>
> > > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > Something seems wrong about this.  The addition of EXPORT_SYMBOL_GPL
> > suggests that such interrupt handlers might live in modules.  In what
> > situation might a module interrupt handler get called from the idle
> > loop, before rcu_irq_enter or after rcu_irq_exit, and need to know that
> > when using RCU?
> 
> Drivers can be in modules, in which case their interrupt handlers will
> also be in the corresponding module.  I do agree that in most cases,
> the irq_enter() and irq_exit() hooks would be invoked by non-module code,
> but I do believe that I had to add those exports due to build failures.

I definitely understand that drivers can have interrupt handlers in
modules; however, I have a hard time seeing a case where a module has a
legitimate reason to invoke rcu_irq_enter or rcu_irq_exit, or to run
before the former or after the latter.

- Josh Triplett
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