Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970

Paul E. McKenney
2012-06-29T22:28:34Z

On Fri, Jun 29, 2012 at 11:40:40PM +0200, Sasha Levin wrote:
> On Fri, 2012-06-29 at 10:23 -0700, Paul E. McKenney wrote:
> > On Fri, Jun 29, 2012 at 12:09:44PM +0200, Sasha Levin wrote:
> > > Hi Paul,
> > >
> > > I think I've stumbled on another bug that will increase your paranoia levels even further.
> > >
> > > I got the following lockup when fuzzing with trinity inside a KVM tools guest, using latest linux-next.
> > >
> > > It appears that it was caused by a03d6178 ("rcu: Move RCU grace-period cleanup into kthread"). This issue doesn't reproduce easily though, it took some fuzzing before hitting it.
> >
> > Hmmm... If the preemption at that point in __rcu_read_unlock() is
> > required to make this happen, then it would be pretty hard to hit.
> > I suspect that you can make it reproduce more quickly by putting
> > a udelay(10) or similar right after the assignment of INT_MIN to
> > t->rcu_read_lock_nesting in __rcu_read_unlock() in kernel/rcupdate.c.
> > Can this be reproduced while running with lockdep enabled?
>
> The good news are that it is much easier to reproduce it by adding a udelay(10) at the point you've mentioned.

How quickly does it reproduce?

> The bad news are that what you saw was a lockdep enabled run (CONFIG_PROVE_RCU is enabled, and lockdep was enabled). There were no lockdep warnings at any point while reproducing it.

Well, at least that means a reasonable way to test fixes.

> > Please see below for an untested patch that gets RCU out of this loop,
> > but it is quite possible that something else is involved here, so it would
> > be very good to get a lockdep run, if possible. My concern stems from the
> > fact that interrupts were enabled during the call to __rcu_read_unlock()
> > -- otherwise it would not have been preempted -- so the runqueue locks
> > were presumably not held on entry to __rcu_read_unlock().
>
> I've scheduled to try this patch tomorrow after a sleep, unless you preempt me first :)

Get some sleep!!!

Thanx, Paul

> > ------------------------------------------------------------------------
> >
> > rcu: Avoid grace-period kthread wakeups from unsafe environments
> >
> > A soft lockup involving the run-queue locks involved the wakeup of
> > the grace-period kthread. This commit therefore defers the wakeup
> > to softirq context.
> >
> > Reported-by: Sasha Levin <levinsasha928@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index c0e0454..cd0076e 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1262,6 +1262,15 @@ static int rcu_gp_kthread(void *arg)
> > }
> >
> > /*
> > + * Signal the RCU core to wake up the grace-period kthread, as we might
> > + * not be in a context where it is safe to do the wakeup directly.
> > + */
> > +static void rcu_wake_gp_kthread(struct rcu_state *rsp)
> > +{
> > + ACCESS_ONCE(rsp->wake_gp_kthread) = 1;
> > +}
> > +
> > +/*
> > * Start a new RCU grace period if warranted, re-initializing the hierarchy
> > * in preparation for detecting the next grace period. The caller must hold
> > * the root node's ->lock, which is released before return. Hard irqs must
> > @@ -1291,7 +1300,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
> >
> > rsp->gp_flags = 1;
> > raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > - wake_up(&rsp->gp_wq);
> > + rcu_wake_gp_kthread(rsp);
> > }
> >
> > /*
> > @@ -1306,7 +1315,7 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
> > {
> > WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
> > raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> > - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
> > + rcu_wake_gp_kthread(rsp); /* smp_mb() implied by wakeup. */
> > }
> >
> > /*
> > @@ -1889,6 +1898,12 @@ __rcu_process_callbacks(struct rcu_state *rsp)
> >
> > WARN_ON_ONCE(rdp->beenonline == 0);
> >
> > + /* Awaken the grace-period kthread if needed. */
> > + if (ACCESS_ONCE(rsp->wake_gp_kthread)) {
> > + if (xchg(&rsp->wake_gp_kthread, 0)) /* Prevent wakeup storms. */
> > + wake_up(&rsp->gp_wq);
> > + }
> > +
> > /*
> > * Advance callbacks in response to end of earlier grace
> > * period that some other CPU ended.
> > @@ -2304,6 +2319,12 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
> > return 1;
> > }
> >
> > + /* Does the grace-period kthread need to be awakened? */
> > + if (ACCESS_ONCE(rsp->wake_gp_kthread)) {
> > + rdp->n_wake_gp_kthread++;
> > + return 1;
> > + }
> > +
> > /* nothing to do */
> > rdp->n_rp_need_nothing++;
> > return 0;
> > diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> > index 280ad7c..89a190c 100644
> > --- a/kernel/rcutree.h
> > +++ b/kernel/rcutree.h
> > @@ -313,6 +313,7 @@ struct rcu_data {
> > unsigned long n_rp_cpu_needs_gp;
> > unsigned long n_rp_gp_completed;
> > unsigned long n_rp_gp_started;
> > + unsigned long n_wake_gp_kthread;
> > unsigned long n_rp_need_nothing;
> >
> > /* 6) _rcu_barrier() callback. */
> > @@ -381,6 +382,7 @@ struct rcu_state {
> > struct task_struct *gp_kthread; /* Task for grace periods. */
> > wait_queue_head_t gp_wq; /* Where GP task waits. */
> > int gp_flags; /* Commands for GP task. */
> > + int wake_gp_kthread; /* Awaken GP task? */
> >
> > /* End of fields guarded by root rcu_node's lock. */
> >
> >
> > --
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