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

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

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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