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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>
hifive-unleashed-5.1
Paul E. McKenney 2012-10-07 08:36:12 -07:00
parent cb349ca954
commit a4fbe35a12
2 changed files with 16 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
kernel/rcutree.c
View File

@ -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, \
}
@ -1197,7 +1198,7 @@ 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
@ -1234,7 +1235,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
cond_resched();
}
put_online_cpus();
mutex_unlock(&rsp->onoff_mutex);
return 1;
}
@ -1700,6 +1701,7 @@ 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. */
@ -1744,6 +1746,7 @@ 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 */
@ -2648,6 +2651,9 @@ 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. */
@ -2662,14 +2668,6 @@ 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;
@ -2693,8 +2691,9 @@ 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)

6
kernel/rcutree.h
View File

@ -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 */