Date:	Sat, 18 Jun 2011 16:23:58 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Milton Miller <miltonm@....com>
Subject: Re: [PATCH 1/4] rcu: Detect uses of rcu read side in extended
 quiescent states

On Fri, Jun 17, 2011 at 04:19:03PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 10, 2011 at 02:50:43AM +0200, Frederic Weisbecker wrote:
> > On Thu, Jun 09, 2011 at 05:23:50PM -0700, Paul E. McKenney wrote:
> > > On Fri, Jun 10, 2011 at 01:47:24AM +0200, Frederic Weisbecker wrote:
> > > > Detect uses of rcu that are not supposed to happen when we
> > > > are in an extended quiescent state.
> > > > 
> > > > This can happen for example if we use rcu between the time we
> > > > stop the tick and the time we restart it. Or inside an irq that
> > > > didn't use rcu_irq_enter,exit() or other possible kind of rcu API
> > > > misuse.
> > > > 
> > > > v2: Rebase against latest rcu changes, handle tiny RCU as well
> > > 
> > > Good idea on checking for RCU read-side critical sections happening
> > > in dyntick-idle periods!
> > > 
> > > But wouldn't it be better to put the checks in rcu_read_lock() and
> > > friends?  The problem I see with putting them in rcu_dereference_check()
> > > is that someone can legitimately do something like the following
> > > while in dyntick-idle mode:
> > > 
> > > 	spin_lock(&mylock);
> > > 	/* do a bunch of stuff */
> > > 	p = rcu_dereference_check(myrcuptr, lockdep_is_held(&mylock));
> > > 
> > > The logic below would complain about this usage, despite the fact
> > > that it is perfectly safe because the update-side lock is held.
> > > 
> > > Make sense, or am I missing something?
> > > 
> > > 						Thanx, Paul
> > 
> > I'm an idiot. I put my check in rcu_dereference_check() on purpose because
> > it's always called from places that check one of the rcu locks are held,
> > but I forgot that's also used for custom conditions with the _check()
> > things.
> > 
> > That said, putting the check in rcu_read_lock() and alike  would only work
> > with rcu_read_lock() itself. Few users of rcu_read_lock_sched() actually
> > call it explicitely but rely on irq disabled or preempt disabled. And I can't put the
> > checks there as it's fine to disabled irqs in dyntick idle.
> > 
> > What about the below? (untested yet)
> > 
> > And I would print the state of dynticks-idle mode in the final lockdep warning.
> 
> Printing the dynticks-idle mode would be quite good!
> 
> However, it is possible to have an RCU read-side critical section that does
> not have an rcu_dereference() or an rcu_read_lock_held().  So I do believe
> that we really do need rcu_read_lock() and friends to do this checking.

Right, then we need to check everything: rcu_read_lock() and friends in case
we have no rcu_read_lock_held() check made (ie: no rcu_dereference_check()),
but also rcu_read_lock_held()/rcu_read_lock_sched_held()/... because preempt_disable(),
local_irq_disable(), local_bh_disable() can't be checked so for rcu sched and rcu bh
we can only check the ...held() things.

> 
> That might seem to leave open the possibility of a stray rcu_dereference()
> being executed from dyntick-idle mode, but the existing PROVE_RCU
> checking will catch that, right?
> 
> So I believe that the simplest approach with the best coverage is to
> put the checks into RCU's read-side critical-section-entry primitives.
> 
> Make sense, or am I confused?

If we also check the rcu_read_...._held() things then yeah that works.
But checking only rcu_read_..._lock() things in not sufficient like I said
above.
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