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Subject[PATCH tip/core/rcu 1/2] rcu: fix grace-period-stall bug on large systems with CPU hotplug
Date
When the last CPU of a given leaf rcu_node structure goes offline,
all of the tasks queued on that leaf rcu_node structure (due to having
blocked in their current RCU read-side critical sections) are requeued
onto the root rcu_node structure. This requeuing is carried out by
rcu_preempt_offline_tasks(). However, it is possible that these queued
tasks are the only thing preventing the leaf rcu_node structure from
reporting a quiescent state up the rcu_node hierarchy. Unfortunately,
the old code would fail to do this reporting, resulting in a grace-period
stall given the following sequence of events:

1. Kernel built for more than 32 CPUs on 32-bit systems or for more
than 64 CPUs on 64-bit systems, so that there is more than one
rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set
to a number smaller than CONFIG_NR_CPUS.)

2. The kernel is built with CONFIG_TREE_PREEMPT_RCU.

3. A task running on a CPU associated with a given leaf rcu_node
structure blocks while in an RCU read-side critical section
-and- that CPU has not yet passed through a quiescent state
for the current RCU grace period. This will cause the task
to be queued on the leaf rcu_node's blocked_tasks[] array, in
particular, on the element of this array corresponding to the
current grace period.

4. Each of the remaining CPUs corresponding to this same leaf rcu_node
structure pass through a quiescent state. However, the task is
still in its RCU read-side critical section, so these quiescent
states cannot be reported further up the rcu_node hierarchy.
Nevertheless, all bits in the leaf rcu_node structure's ->qsmask
field are now zero.

5. Each of the remaining CPUs go offline. (The events in step
#4 and #5 can happen in any order as long as each CPU passes
through a quiescent state before going offline.)

6. When the last CPU goes offline, __rcu_offline_cpu() will invoke
rcu_preempt_offline_tasks(), which will move the task to the
root rcu_node structure, but without reporting a quiescent state
up the rcu_node hierarchy (and this failure to report a quiescent
state is the bug).

But because this leaf rcu_node structure's ->qsmask field is
already zero and its ->block_tasks[] entries are all empty,
force_quiescent_state() will skip this rcu_node structure.

Therefore, grace periods are now hung.

This patch abstracts some code out of rcu_read_unlock_special(), calling
the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet()
from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking
task_quiet() from __rcu_offline_cpu() reports the quiescent state up
the rcu_node hierarchy, fixing the bug. This ends up requiring a separate
lock_class_key per level of the rcu_node hierarchy, which this patch also
provides.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 40 +++++++++++----------
kernel/rcutree.h | 3 ++
kernel/rcutree_plugin.h | 85 +++++++++++++++++++++++++++++++++-------------
3 files changed, 85 insertions(+), 43 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 9b36d6d..b79bfcd 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -51,7 +51,7 @@

/* Data structures. */

-static struct lock_class_key rcu_root_class;
+static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];

#define RCU_STATE_INITIALIZER(name) { \
.level = { &name.node[0] }, \
@@ -936,6 +936,7 @@ static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
{
unsigned long flags;
unsigned long mask;
+ int need_quiet = 0;
struct rcu_data *rdp = rsp->rda[cpu];
struct rcu_node *rnp;

@@ -949,29 +950,30 @@ static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
spin_lock(&rnp->lock); /* irqs already disabled. */
rnp->qsmaskinit &= ~mask;
if (rnp->qsmaskinit != 0) {
- spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ if (rnp != rdp->mynode)
+ spin_unlock(&rnp->lock); /* irqs remain disabled. */
break;
}
-
- /*
- * If there was a task blocking the current grace period,
- * and if all CPUs have checked in, we need to propagate
- * the quiescent state up the rcu_node hierarchy. But that
- * is inconvenient at the moment due to deadlock issues if
- * this should end the current grace period. So set the
- * offlined CPU's bit in ->qsmask in order to force the
- * next force_quiescent_state() invocation to clean up this
- * mess in a deadlock-free manner.
- */
- if (rcu_preempt_offline_tasks(rsp, rnp, rdp) && !rnp->qsmask)
- rnp->qsmask |= mask;
-
+ if (rnp == rdp->mynode)
+ need_quiet = rcu_preempt_offline_tasks(rsp, rnp, rdp);
+ else
+ spin_unlock(&rnp->lock); /* irqs remain disabled. */
mask = rnp->grpmask;
- spin_unlock(&rnp->lock); /* irqs remain disabled. */
rnp = rnp->parent;
} while (rnp != NULL);

- spin_unlock_irqrestore(&rsp->onofflock, flags);
+ /*
+ * We still hold the leaf rcu_node structure lock here, and
+ * irqs are still disabled. The reason for this subterfuge is
+ * because invoking task_quiet() with ->onofflock held leads
+ * to deadlock.
+ */
+ spin_unlock(&rsp->onofflock); /* irqs remain disabled. */
+ rnp = rdp->mynode;
+ if (need_quiet)
+ task_quiet(rnp, flags);
+ else
+ spin_unlock_irqrestore(&rnp->lock, flags);

rcu_adopt_orphan_cbs(rsp);
}
@@ -1731,6 +1733,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
rnp = rsp->level[i];
for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
spin_lock_init(&rnp->lock);
+ lockdep_set_class(&rnp->lock, &rcu_node_class[i]);
rnp->gpnum = 0;
rnp->qsmask = 0;
rnp->qsmaskinit = 0;
@@ -1753,7 +1756,6 @@ static void __init rcu_init_one(struct rcu_state *rsp)
INIT_LIST_HEAD(&rnp->blocked_tasks[1]);
}
}
- lockdep_set_class(&rcu_get_root(rsp)->lock, &rcu_root_class);
}

/*
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 17a28a0..a81188c 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -305,6 +305,9 @@ static void rcu_bootup_announce(void);
long rcu_batches_completed(void);
static void rcu_preempt_note_context_switch(int cpu);
static int rcu_preempted_readers(struct rcu_node *rnp);
+#ifdef CONFIG_HOTPLUG_CPU
+static void task_quiet(struct rcu_node *rnp, unsigned long flags);
+#endif /* #ifdef CONFIG_HOTPLUG_CPU */
#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
static void rcu_print_task_stall(struct rcu_node *rnp);
#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 5ca2d26..0bdb592 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -160,11 +160,51 @@ static int rcu_preempted_readers(struct rcu_node *rnp)
return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
}

+/*
+ * Record a quiescent state for all tasks that were previously queued
+ * on the specified rcu_node structure and that were blocking the current
+ * RCU grace period. The caller must hold the specified rnp->lock with
+ * irqs disabled, and this lock is released upon return, but irqs remain
+ * disabled.
+ */
+static void task_quiet(struct rcu_node *rnp, unsigned long flags)
+ __releases(rnp->lock)
+{
+ unsigned long mask;
+ struct rcu_node *rnp_p;
+
+ if (rnp->qsmask != 0 || rcu_preempted_readers(rnp)) {
+ spin_unlock_irqrestore(&rnp->lock, flags);
+ return; /* Still need more quiescent states! */
+ }
+
+ rnp_p = rnp->parent;
+ if (rnp_p == NULL) {
+ /*
+ * Either there is only one rcu_node in the tree,
+ * or tasks were kicked up to root rcu_node due to
+ * CPUs going offline.
+ */
+ cpu_quiet_msk_finish(&rcu_preempt_state, flags);
+ return;
+ }
+
+ /* Report up the rest of the hierarchy. */
+ mask = rnp->grpmask;
+ spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ spin_lock(&rnp_p->lock); /* irqs already disabled. */
+ cpu_quiet_msk(mask, &rcu_preempt_state, rnp_p, flags);
+}
+
+/*
+ * Handle special cases during rcu_read_unlock(), such as needing to
+ * notify RCU core processing or task having blocked during the RCU
+ * read-side critical section.
+ */
static void rcu_read_unlock_special(struct task_struct *t)
{
int empty;
unsigned long flags;
- unsigned long mask;
struct rcu_node *rnp;
int special;

@@ -213,30 +253,15 @@ static void rcu_read_unlock_special(struct task_struct *t)
/*
* If this was the last task on the current list, and if
* we aren't waiting on any CPUs, report the quiescent state.
- * Note that both cpu_quiet_msk_finish() and cpu_quiet_msk()
- * drop rnp->lock and restore irq.
+ * Note that task_quiet() releases rnp->lock.
*/
- if (!empty && rnp->qsmask == 0 &&
- !rcu_preempted_readers(rnp)) {
- struct rcu_node *rnp_p;
-
- if (rnp->parent == NULL) {
- /* Only one rcu_node in the tree. */
- cpu_quiet_msk_finish(&rcu_preempt_state, flags);
- return;
- }
- /* Report up the rest of the hierarchy. */
- mask = rnp->grpmask;
+ if (empty)
spin_unlock_irqrestore(&rnp->lock, flags);
- rnp_p = rnp->parent;
- spin_lock_irqsave(&rnp_p->lock, flags);
- WARN_ON_ONCE(rnp->qsmask);
- cpu_quiet_msk(mask, &rcu_preempt_state, rnp_p, flags);
- return;
- }
- spin_unlock(&rnp->lock);
+ else
+ task_quiet(rnp, flags);
+ } else {
+ local_irq_restore(flags);
}
- local_irq_restore(flags);
}

/*
@@ -303,6 +328,8 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
* rcu_node. The reason for not just moving them to the immediate
* parent is to remove the need for rcu_read_unlock_special() to
* make more than two attempts to acquire the target rcu_node's lock.
+ * Returns true if there were tasks blocking the current RCU grace
+ * period.
*
* Returns 1 if there was previously a task blocking the current grace
* period on the specified rcu_node structure.
@@ -316,7 +343,7 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
int i;
struct list_head *lp;
struct list_head *lp_root;
- int retval = rcu_preempted_readers(rnp);
+ int retval;
struct rcu_node *rnp_root = rcu_get_root(rsp);
struct task_struct *tp;

@@ -334,6 +361,7 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
* rcu_nodes in terms of gp_num value. This fact allows us to
* move the blocked_tasks[] array directly, element by element.
*/
+ retval = rcu_preempted_readers(rnp);
for (i = 0; i < 2; i++) {
lp = &rnp->blocked_tasks[i];
lp_root = &rnp_root->blocked_tasks[i];
@@ -346,7 +374,6 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
spin_unlock(&rnp_root->lock); /* irqs remain disabled */
}
}
-
return retval;
}

@@ -512,6 +539,16 @@ static int rcu_preempted_readers(struct rcu_node *rnp)
return 0;
}

+#ifdef CONFIG_HOTPLUG_CPU
+
+/* Because preemptible RCU does not exist, no quieting of tasks. */
+static void task_quiet(struct rcu_node *rnp, unsigned long flags)
+{
+ spin_unlock_irqrestore(&rnp->lock, flags);
+}
+
+#endif /* #ifdef CONFIG_HOTPLUG_CPU */
+
#ifdef CONFIG_RCU_CPU_STALL_DETECTOR

/*
--
1.5.2.5


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