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Date: Fri, 15 Jul 2011 13:16:44 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: paulmck@...ux.vnet.ibm.com
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>, Ed Tomlinson <edt@....ca>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Dipankar Sarma <dipankar@...ibm.com>,
linux-kernel@...r.kernel.org
Subject: Re: INFO: possible circular locking dependency detected
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 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.
We will probably need to put a preempt_disable() in there too, to keep
the ->special being zero and scheduled out.
>
> 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?
>
> > + 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()?
-- 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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