Re: Deadlock on poweroff

Paul E. McKenney
2012-10-07T21:08:21Z

On Sun, Oct 07, 2012 at 10:46:54PM +0530, Srivatsa S. Bhat wrote:
> On 10/07/2012 10:41 PM, Kirill A. Shutemov wrote:
> > On Sun, Oct 07, 2012 at 10:35:01PM +0530, Srivatsa S. Bhat wrote:
> >> On 10/07/2012 10:20 PM, Kirill A. Shutemov wrote:
> >>> On Sun, Oct 07, 2012 at 09:03:11AM -0700, Paul E. McKenney wrote:
> >>>> On Sun, Oct 07, 2012 at 05:47:11AM +0300, Kirill A. Shutemov wrote:
> >>>>> Hi Paul and all,
> >>>>>
> >>>>> With commit 755609a9087fa983f567dc5452b2fa7b089b591f I've got deadlock on
> >>>>> poweroff.
> >>>>>
> >>>>> It guess it happens because of race for cpu_hotplug.lock:
> >>>>>
> >>>>> CPU A CPU B
> >>>>> disable_nonboot_cpus()
> >>>>> _cpu_down()
> >>>>> cpu_hotplug_begin()
> >>>>> mutex_lock(&cpu_hotplug.lock);
> >>>>> __cpu_notify()
> >>>>> padata_cpu_callback()
> >>>>> __padata_remove_cpu()
> >>>>> padata_replace()
> >>>>> synchronize_rcu()
> >>>>> rcu_gp_kthread()
> >>>>> get_online_cpus();
> >>>>> mutex_lock(&cpu_hotplug.lock);
> >>>>>
> >>>>> Have you seen the issue before?
> >>>>
> >>>> This is a new one for me. Does the following (very lightly tested)
> >>>> patch help?
> >>>
> >>> Works for me. Thanks.
> >>>
> >>
> >> Could you share the patch please? Looks like it didn't hit the mailing
> >> lists..
> >
> > Sure. Here's original mail from Paul:
>
> Ah, great! Thanks!

Thank you, Kirill.

I wonder what the mailing list doesn't like about me... ;-)

Thanx, Paul

> Regards,
> Srivatsa S. Bhat
>
> > Date: Sun, 7 Oct 2012 09:03:11 -0700
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > To: "Kirill A. Shutemov" <kirill@shutemov.name>
> > Cc: linux-kernel@vger.kernel.org, Dipankar Sarma <dipankar@in.ibm.com>,
> > Thomas Gleixner <tglx@linutronix.de>,
> > Andrew Morton <akpm@linux-foundation.org>,
> > Steffen Klassert <steffen.klassert@secunet.com>,
> > ".linux-crypto"@vger.kernel.org
> > Subject: Re: Deadlock on poweroff
> > Message-ID: <20121007160311.GE2485@linux.vnet.ibm.com>
> > Reply-To: paulmck@linux.vnet.ibm.com
> > References: <20121007024711.GA21403@shutemov.name>
> > MIME-Version: 1.0
> > Content-Type: text/plain; charset=us-ascii
> > Content-Disposition: inline
> > In-Reply-To: <20121007024711.GA21403@shutemov.name>
> > User-Agent: Mutt/1.5.21 (2010-09-15)
> > X-Content-Scanned: Fidelis XPS MAILER
> > x-cbid: 12100716-7408-0000-0000-000009194B17
> > Status: RO
> > Content-Length: 6055
> > Lines: 173
> >
> > On Sun, Oct 07, 2012 at 05:47:11AM +0300, Kirill A. Shutemov wrote:
> >> Hi Paul and all,
> >>
> >> With commit 755609a9087fa983f567dc5452b2fa7b089b591f I've got deadlock on
> >> poweroff.
> >>
> >> It guess it happens because of race for cpu_hotplug.lock:
> >>
> >> CPU A CPU B
> >> disable_nonboot_cpus()
> >> _cpu_down()
> >> cpu_hotplug_begin()
> >> mutex_lock(&cpu_hotplug.lock);
> >> __cpu_notify()
> >> padata_cpu_callback()
> >> __padata_remove_cpu()
> >> padata_replace()
> >> synchronize_rcu()
> >> rcu_gp_kthread()
> >> get_online_cpus();
> >> mutex_lock(&cpu_hotplug.lock);
> >>
> >> Have you seen the issue before?
> >
> > This is a new one for me. Does the following (very lightly tested)
> > patch help?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > rcu: Grace-period initialization excludes only RCU notifier
> >
> > Kirill noted the following deadlock cycle on shutdown involving padata:
> >
> >> With commit 755609a9087fa983f567dc5452b2fa7b089b591f I've got deadlock on
> >> poweroff.
> >>
> >> It guess it happens because of race for cpu_hotplug.lock:
> >>
> >> CPU A CPU B
> >> disable_nonboot_cpus()
> >> _cpu_down()
> >> cpu_hotplug_begin()
> >> mutex_lock(&cpu_hotplug.lock);
> >> __cpu_notify()
> >> padata_cpu_callback()
> >> __padata_remove_cpu()
> >> padata_replace()
> >> synchronize_rcu()
> >> rcu_gp_kthread()
> >> get_online_cpus();
> >> mutex_lock(&cpu_hotplug.lock);
> >
> > It would of course be good to eliminate grace-period delays from
> > CPU-hotplug notifiers, but that is a separate issue. Deadlock is
> > not an appropriate diagnostic for excessive CPU-hotplug latency.
> >
> > Fortunately, grace-period initialization does not actually need to
> > exclude all of the CPU-hotplug operation, but rather only RCU's own
> > CPU_UP_PREPARE and CPU_DEAD CPU-hotplug notifiers. This commit therefore
> > introduces a new per-rcu_state onoff_mutex that provides the required
> > concurrency control in place of the get_online_cpus() that was previously
> > in rcu_gp_init().
> >
> > Reported-by: "Kirill A. Shutemov" <kirill@shutemov.name>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index fb63d7b..5eece12 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -74,6 +74,7 @@ static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
> > .orphan_nxttail = &sname##_state.orphan_nxtlist, \
> > .orphan_donetail = &sname##_state.orphan_donelist, \
> > .barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
> > + .onoff_mutex = __MUTEX_INITIALIZER(sname##_state.onoff_mutex), \
> > .name = #sname, \
> > }
> >
> > @@ -1229,7 +1230,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
> > raw_spin_unlock_irq(&rnp->lock);
> >
> > /* Exclude any concurrent CPU-hotplug operations. */
> > - get_online_cpus();
> > + mutex_lock(&rsp->onoff_mutex);
> >
> > /*
> > * Set the quiescent-state-needed bits in all the rcu_node
> > @@ -1266,7 +1267,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
> > cond_resched();
> > }
> >
> > - put_online_cpus();
> > + mutex_unlock(&rsp->onoff_mutex);
> > return 1;
> > }
> >
> > @@ -1754,6 +1755,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
> > /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
> >
> > /* Exclude any attempts to start a new grace period. */
> > + mutex_lock(&rsp->onoff_mutex);
> > raw_spin_lock_irqsave(&rsp->onofflock, flags);
> >
> > /* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
> > @@ -1798,6 +1800,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
> > init_callback_list(rdp);
> > /* Disallow further callbacks on this CPU. */
> > rdp->nxttail[RCU_NEXT_TAIL] = NULL;
> > + mutex_unlock(&rsp->onoff_mutex);
> > }
> >
> > #else /* #ifdef CONFIG_HOTPLUG_CPU */
> > @@ -2708,6 +2711,9 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> > struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
> > struct rcu_node *rnp = rcu_get_root(rsp);
> >
> > + /* Exclude new grace periods. */
> > + mutex_lock(&rsp->onoff_mutex);
> > +
> > /* Set up local state, ensuring consistent view of global state. */
> > raw_spin_lock_irqsave(&rnp->lock, flags);
> > rdp->beenonline = 1; /* We have now been online. */
> > @@ -2722,14 +2728,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> > rcu_prepare_for_idle_init(cpu);
> > raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> >
> > - /*
> > - * A new grace period might start here. If so, we won't be part
> > - * of it, but that is OK, as we are currently in a quiescent state.
> > - */
> > -
> > - /* Exclude any attempts to start a new GP on large systems. */
> > - raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */
> > -
> > /* Add CPU to rcu_node bitmasks. */
> > rnp = rdp->mynode;
> > mask = rdp->grpmask;
> > @@ -2753,8 +2751,9 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> > raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
> > rnp = rnp->parent;
> > } while (rnp != NULL && !(rnp->qsmaskinit & mask));
> > + local_irq_restore(flags);
> >
> > - raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> > + mutex_unlock(&rsp->onoff_mutex);
> > }
> >
> > static void __cpuinit rcu_prepare_cpu(int cpu)
> > diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> > index 5faf05d..a240f03 100644
> > --- a/kernel/rcutree.h
> > +++ b/kernel/rcutree.h
> > @@ -394,11 +394,17 @@ struct rcu_state {
> > struct rcu_head **orphan_donetail; /* Tail of above. */
> > long qlen_lazy; /* Number of lazy callbacks. */
> > long qlen; /* Total number of callbacks. */
> > + /* End of fields guarded by onofflock. */
> > +
> > + struct mutex onoff_mutex; /* Coordinate hotplug & GPs. */
> > +
> > struct mutex barrier_mutex; /* Guards barrier fields. */
> > atomic_t barrier_cpu_count; /* # CPUs waiting on. */
> > struct completion barrier_completion; /* Wake at barrier end. */
> > unsigned long n_barrier_done; /* ++ at start and end of */
> > /* _rcu_barrier(). */
> > + /* End of fields guarded by barrier_mutex. */
> > +
> > unsigned long jiffies_force_qs; /* Time at which to invoke */
> > /* force_quiescent_state(). */
> > unsigned long n_force_qs; /* Number of calls to */
> >
>