Messages in this thread | | | Subject | Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970 | From | Sasha Levin <> | Date | Fri, 29 Jun 2012 23:40:40 +0200 |
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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.
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.
> 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 :)
> 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. */ > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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