Re: [PATCH 1/4] rcu: Detect uses of rcu read side in extended quiescent states

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Jun 18, 2011 at 04:23:58PM +0200, Frederic Weisbecker wrote:
> 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.

Good point.  To make sure I understand, we have different approaches
for the different types of RCU.  We need to instrument the following
primitives:

1.	rcu_read_lock() because a stray rcu_dereference() will already
	be caught by PROVE_RCU.
2.	rcu_read_lock_bh_held() because it is OK to do local_bh_disable()
	in dyntick-idle mode, so we cannot prohibit all of the
	read-acquisition cases.  We can also instrument rcu_read_lock_bh()
	to catch RCU-bh read-side critical sections that don't happen
	to contain rcu_dereference_bh().
3.	rcu_read_lock_sched_held() because it is OK to do preempt_disable()
	and local_irq_save() from dyntick-idle mode, so we again
	cannot prohibit all of the read-acquisition cases.  We can
	also instrument rcu_read_lock_sched() to catch RCU-sched
	read-side critical sections that don't happen to contain
	rcu_dereference_sched().
4.	srcu_read_lock() because a stray srcu_dereference() will already
	by caught by PROVE_RCU.

We miss a few cases, for example, an RCU-sched read-side critical
section that uses local_irq_disable(), but that also does not contain
an rcu_dereference_sched().  But still this sounds quite worthwhile.

> > 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.

Got it!

							Thanx, Paul