Re: Deadlock on poweroff

Kirill A. Shutemov
2012-10-07T17:10:31Z

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:

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>
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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 */