rcu: Grace-period initialization excludes only RCU notifier
Kirill noted the following deadlock cycle on shutdown involving padata:
> With commit 755609a908
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>
Tested-by: Kirill A. Shutemov <kirill@shutemov.name>
This commit is contained in:
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@ -74,6 +74,7 @@ static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
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.orphan_nxttail = &sname##_state.orphan_nxtlist, \
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.orphan_donetail = &sname##_state.orphan_donelist, \
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.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
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.onoff_mutex = __MUTEX_INITIALIZER(sname##_state.onoff_mutex), \
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.name = #sname, \
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}
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@ -1197,7 +1198,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
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raw_spin_unlock_irq(&rnp->lock);
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/* Exclude any concurrent CPU-hotplug operations. */
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get_online_cpus();
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mutex_lock(&rsp->onoff_mutex);
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/*
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* Set the quiescent-state-needed bits in all the rcu_node
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@ -1234,7 +1235,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
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cond_resched();
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}
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put_online_cpus();
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mutex_unlock(&rsp->onoff_mutex);
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return 1;
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}
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@ -1700,6 +1701,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
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/* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
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/* Exclude any attempts to start a new grace period. */
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mutex_lock(&rsp->onoff_mutex);
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raw_spin_lock_irqsave(&rsp->onofflock, flags);
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/* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
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@ -1744,6 +1746,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
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init_callback_list(rdp);
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/* Disallow further callbacks on this CPU. */
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rdp->nxttail[RCU_NEXT_TAIL] = NULL;
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mutex_unlock(&rsp->onoff_mutex);
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}
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#else /* #ifdef CONFIG_HOTPLUG_CPU */
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@ -2648,6 +2651,9 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
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struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
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struct rcu_node *rnp = rcu_get_root(rsp);
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/* Exclude new grace periods. */
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mutex_lock(&rsp->onoff_mutex);
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/* Set up local state, ensuring consistent view of global state. */
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raw_spin_lock_irqsave(&rnp->lock, flags);
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rdp->beenonline = 1; /* We have now been online. */
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@ -2662,14 +2668,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
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rcu_prepare_for_idle_init(cpu);
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raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
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/*
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* A new grace period might start here. If so, we won't be part
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* of it, but that is OK, as we are currently in a quiescent state.
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*/
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/* Exclude any attempts to start a new GP on large systems. */
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raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */
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/* Add CPU to rcu_node bitmasks. */
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rnp = rdp->mynode;
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mask = rdp->grpmask;
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@ -2693,8 +2691,9 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
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raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
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rnp = rnp->parent;
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} while (rnp != NULL && !(rnp->qsmaskinit & mask));
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local_irq_restore(flags);
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raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
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mutex_unlock(&rsp->onoff_mutex);
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}
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static void __cpuinit rcu_prepare_cpu(int cpu)
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@ -394,11 +394,17 @@ struct rcu_state {
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struct rcu_head **orphan_donetail; /* Tail of above. */
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long qlen_lazy; /* Number of lazy callbacks. */
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long qlen; /* Total number of callbacks. */
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/* End of fields guarded by onofflock. */
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struct mutex onoff_mutex; /* Coordinate hotplug & GPs. */
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struct mutex barrier_mutex; /* Guards barrier fields. */
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atomic_t barrier_cpu_count; /* # CPUs waiting on. */
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struct completion barrier_completion; /* Wake at barrier end. */
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unsigned long n_barrier_done; /* ++ at start and end of */
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/* _rcu_barrier(). */
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/* End of fields guarded by barrier_mutex. */
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unsigned long jiffies_force_qs; /* Time at which to invoke */
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/* force_quiescent_state(). */
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unsigned long n_force_qs; /* Number of calls to */
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