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SubjectRe: INFO: possible circular locking dependency detected
On Fri, Jul 15, 2011 at 01:16:44PM -0400, Steven Rostedt wrote:
> On Fri, 2011-07-15 at 10:03 -0700, Paul E. McKenney wrote:
> > On Fri, Jul 15, 2011 at 12:55:57PM -0400, Steven Rostedt wrote:
> > > On Fri, 2011-07-15 at 15:07 +0200, Peter Zijlstra wrote:
> > >
> > > > OK, so the latter case cannot happen (rcu_preempt_check_callbacks only
> > > > sets NEED_QS when rcu_read_lock_nesting), we need two interrupts for
> > > > this to happen.
> > > >
> > > > rcu_read_lock()
> > > >
> > > > <IRQ>
> > > > |= RCU_READ_UNLOCK_NEED_QS
> > > >
> > > > rcu_read_unlock()
> > > > __rcu_read_unlock()
> > > > --rcu_read_lock_nesting;
> > > > <IRQ>
> > > > ttwu()
> > > > rcu_read_lock()
> > > > rcu_read_unlock()
> > > > rcu_read_unlock_special()
> > > > *BANG*
> > > > rcu_read_unlock_special()
> > > >
> > >
> > > What about this patch? Not even compiled tested.
> >
> > This runs afoul of the restriction that ->rcu_read_unlock_special must
> > be updated with irqs disabled, please see below.
>
> What about changing special into a local_t, then it could be updated
> atomically wrt interrupts (not for other CPUs).

I would like to avoid increasing the cost of the rcu_read_unlock()
fastpath. I still believe that it is possible to fix this without
increasing that cost.

> > I am also missing what the goal is -- I don't immediatly see how this
> > prevents the scenario that Ed ran into, for example.
>
> >From the example that Peter showed above:
>
> The interrupt happens after decrementing lock_nesting, and then when it
> did the rcu_read_unlock(), it would call special() because the ->special
> variable was set. My patch makes it so that ->special will *not* be set.

But the rcu_read_unlock() called from within the irq handler would
take a second snapshot of ->special. It could then enter
rcu_read_unlock_special().

> We will probably need to put a preempt_disable() in there too, to keep
> the ->special being zero and scheduled out.

But ->rcu_read_unlock_special is in the task structure, so would move
with the task. But yes, that sort of thing is one reason that I would
like to keep ->rcu_read_unlock_special modifications under irq-disable.

> > Thanx, Paul
> >
> > > -- Steve
> > >
> > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > > index 14dc7dd..e3545fa 100644
> > > --- a/kernel/rcutree_plugin.h
> > > +++ b/kernel/rcutree_plugin.h
> > > @@ -284,18 +284,17 @@ static struct list_head *rcu_next_node_entry(struct task_struct *t,
> > > * notify RCU core processing or task having blocked during the RCU
> > > * read-side critical section.
> > > */
> > > -static void rcu_read_unlock_special(struct task_struct *t)
> > > +static int rcu_read_unlock_special(struct task_struct *t, int special)
> > > {
> > > int empty;
> > > int empty_exp;
> > > unsigned long flags;
> > > struct list_head *np;
> > > struct rcu_node *rnp;
> > > - int special;
> > >
> > > /* NMI handlers cannot block and cannot safely manipulate state. */
> > > if (in_nmi())
> > > - return;
> > > + return special;
> > >
> > > local_irq_save(flags);
> > >
> > > @@ -303,7 +302,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > > * If RCU core is waiting for this CPU to exit critical section,
> > > * let it know that we have done so.
> > > */
> > > - special = t->rcu_read_unlock_special;
> > > if (special & RCU_READ_UNLOCK_NEED_QS) {
> > > rcu_preempt_qs(smp_processor_id());
> > > }
> > > @@ -311,7 +309,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > > /* Hardware IRQ handlers cannot block. */
> > > if (in_irq()) {
> > > local_irq_restore(flags);
> > > - return;
> > > + return special;
> > > }
> > >
> > > /* Clean up if blocked during RCU read-side critical section. */
> > > @@ -373,6 +371,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > > } else {
> > > local_irq_restore(flags);
> > > }
> > > + return special;
> > > }
> > >
> > > /*
> > > @@ -385,13 +384,21 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > > void __rcu_read_unlock(void)
> > > {
> > > struct task_struct *t = current;
> > > + int special;
> > >
> > > + special = ACCESS_ONCE(t->rcu_read_unlock_special);
> > > + /*
> > > + * Clear special here to prevent interrupts from seeing it
> > > + * enabled after decrementing lock_nesting and calling
> > > + * rcu_read_unlock_special().
> > > + */
> >
> > Any change to ->rcu_read_unlock_special from an irq handler that happens
> > here is lost. Changes to ->rcu_read_unlock_special must be done with
> > irqs disabled. And I hope to avoid irq disabling on the rcu_read_unlock()
> > fastpath.
>
> We can check if special changed afterwards. Hmm, would a xchg be bad to
> do?

I would really like to avoid that in the common rcu_read_unlock() fastpath.

> > > + t->rcu_read_unlock_special = 0;
> > > barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */
> > > --t->rcu_read_lock_nesting;
> > > barrier(); /* decrement before load of ->rcu_read_unlock_special */
> > > - if (t->rcu_read_lock_nesting == 0 &&
> > > - unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> > > - rcu_read_unlock_special(t);
> > > + if (t->rcu_read_lock_nesting == 0 && special)
> > > + special = rcu_read_unlock_special(t, special);
> >
> > And changes to ->rcu_read_unlock_special from an irq handler that happens
> > here are also lost.
>
> How expensive is xchg?
>
> special = xchg(&t->rcu_read_lock_special, 0);
> [..]
> special = xchg(&t->rcu_read_lock_special, special);
> /* check special */
>
> Or is xchg too expensive for rcu_read_unlock()?

It is a bit expensive for that fastpath.

Thanx, Paul

> -- Steve
>
> >
> > > + t->rcu_read_unlock_special = special;
> > > #ifdef CONFIG_PROVE_LOCKING
> > > WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
> > > #endif /* #ifdef CONFIG_PROVE_LOCKING */
> > >
> > >
>
>
> --
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