rcu: Fix grace-period-stall bug on large systems with CPU hotplug

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>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12589088301770-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 85 insertions, 43 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 9b36d6d7fb97..b79bfcd28e95 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 17a28a08b559..a81188c42929 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 5ca2d26c5971..0bdb592eee66 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
 
 /*