rcu: Rework detection of use of RCU by offline CPUs

Because newly offlined CPUs continue executing after completing the
CPU_DYING notifiers, they legitimately enter the scheduler and use
RCU while appearing to be offline.  This calls for a more sophisticated
approach as follows:

1.	RCU marks the CPU online during the CPU_UP_PREPARE phase.

2.	RCU marks the CPU offline during the CPU_DEAD phase.

3.	Diagnostics regarding use of read-side RCU by offline CPUs use
	RCU's accounting rather than the cpu_online_map.  (Note that
	__call_rcu() still uses cpu_online_map to detect illegal
	invocations within CPU_DYING notifiers.)

4.	Offline CPUs are prevented from hanging the system by
	force_quiescent_state(), which pays attention to cpu_online_map.
	Some additional work (in a later commit) will be needed to
	guarantee that force_quiescent_state() waits a full jiffy before
	assuming that a CPU is offline, for example, when called from
	idle entry.  (This commit also makes the one-jiffy wait
	explicit, since the old-style implicit wait can now be defeated
	by RCU_FAST_NO_HZ and by rcutorture.)

This approach avoids the false positives encountered when attempting to
use more exact classification of CPU online/offline state.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 67 insertions, 46 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 05470d4caba3..708469a06860 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -320,25 +320,18 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
 static int rcu_implicit_offline_qs(struct rcu_data *rdp)
 {
         /*
-         * If the CPU is offline, it is in a quiescent state.  We can
-         * trust its state not to change because interrupts are disabled.
+         * If the CPU is offline for more than a jiffy, it is in a quiescent
+         * state.  We can trust its state not to change because interrupts
+         * are disabled.  The reason for the jiffy's worth of slack is to
+         * handle CPUs initializing on the way up and finding their way
+         * to the idle loop on the way down.
          */
-        if (cpu_is_offline(rdp->cpu)) {
+        if (cpu_is_offline(rdp->cpu) &&
+            ULONG_CMP_LT(rdp->rsp->gp_start + 2, jiffies)) {
                 trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
                 rdp->offline_fqs++;
                 return 1;
         }
-
-        /*
-         * The CPU is online, so send it a reschedule IPI.  This forces
-         * it through the scheduler, and (inefficiently) also handles cases
-         * where idle loops fail to inform RCU about the CPU being idle.
-         */
-        if (rdp->cpu != smp_processor_id())
-                smp_send_reschedule(rdp->cpu);
-        else
-                set_need_resched();
-        rdp->resched_ipi++;
         return 0;
 }
 
@@ -601,19 +594,33 @@ EXPORT_SYMBOL(rcu_is_cpu_idle);
  * this task being preempted, its old CPU being taken offline, resuming
  * on some other CPU, then determining that its old CPU is now offline.
  * It is OK to use RCU on an offline processor during initial boot, hence
- * the check for rcu_scheduler_fully_active.
+ * the check for rcu_scheduler_fully_active.  Note also that it is OK
+ * for a CPU coming online to use RCU for one jiffy prior to marking itself
+ * online in the cpu_online_mask.  Similarly, it is OK for a CPU going
+ * offline to continue to use RCU for one jiffy after marking itself
+ * offline in the cpu_online_mask.  This leniency is necessary given the
+ * non-atomic nature of the online and offline processing, for example,
+ * the fact that a CPU enters the scheduler after completing the CPU_DYING
+ * notifiers.
+ *
+ * This is also why RCU internally marks CPUs online during the
+ * CPU_UP_PREPARE phase and offline during the CPU_DEAD phase.
  *
  * Disable checking if in an NMI handler because we cannot safely report
  * errors from NMI handlers anyway.
  */
 bool rcu_lockdep_current_cpu_online(void)
 {
+        struct rcu_data *rdp;
+        struct rcu_node *rnp;
         bool ret;
 
         if (in_nmi())
                 return 1;
         preempt_disable();
-        ret = cpu_online(smp_processor_id()) ||
+        rdp = &__get_cpu_var(rcu_sched_data);
+        rnp = rdp->mynode;
+        ret = (rdp->grpmask & rnp->qsmaskinit) ||
               !rcu_scheduler_fully_active;
         preempt_enable();
         return ret;
@@ -1308,14 +1315,12 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
  */
 static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
 {
-        unsigned long flags;
         int i;
         unsigned long mask;
-        int need_report;
         int receive_cpu = cpumask_any(cpu_online_mask);
         struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
         struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
-        struct rcu_node *rnp = rdp->mynode; /* For dying CPU. */
+        RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */
 
         /* First, adjust the counts. */
         if (rdp->nxtlist != NULL) {
@@ -1381,32 +1386,6 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
                                "cpuofl");
         rcu_report_qs_rdp(smp_processor_id(), rsp, rdp, rsp->gpnum);
         /* Note that rcu_report_qs_rdp() might call trace_rcu_grace_period(). */
-
-        /*
-         * Remove the dying CPU from the bitmasks in the rcu_node
-         * hierarchy.  Because we are in stop_machine() context, we
-         * automatically exclude ->onofflock critical sections.
-         */
-        do {
-                raw_spin_lock_irqsave(&rnp->lock, flags);
-                rnp->qsmaskinit &= ~mask;
-                if (rnp->qsmaskinit != 0) {
-                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                        break;
-                }
-                if (rnp == rdp->mynode) {
-                        need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
-                        if (need_report & RCU_OFL_TASKS_NORM_GP)
-                                rcu_report_unblock_qs_rnp(rnp, flags);
-                        else
-                                raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                        if (need_report & RCU_OFL_TASKS_EXP_GP)
-                                rcu_report_exp_rnp(rsp, rnp, true);
-                } else
-                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                mask = rnp->grpmask;
-                rnp = rnp->parent;
-        } while (rnp != NULL);
 }
 
 /*
@@ -1417,11 +1396,53 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
  */
 static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
 {
+        unsigned long flags;
+        unsigned long mask;
+        int need_report = 0;
         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
-        struct rcu_node *rnp = rdp->mynode;
+        struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rnp. */
 
+        /* Adjust any no-longer-needed kthreads. */
         rcu_stop_cpu_kthread(cpu);
         rcu_node_kthread_setaffinity(rnp, -1);
+
+        /* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
+
+        /* Exclude any attempts to start a new grace period. */
+        raw_spin_lock_irqsave(&rsp->onofflock, flags);
+
+        /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
+        mask = rdp->grpmask;    /* rnp->grplo is constant. */
+        do {
+                raw_spin_lock(&rnp->lock);      /* irqs already disabled. */
+                rnp->qsmaskinit &= ~mask;
+                if (rnp->qsmaskinit != 0) {
+                        if (rnp != rdp->mynode)
+                                raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+                        break;
+                }
+                if (rnp == rdp->mynode)
+                        need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
+                else
+                        raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+                mask = rnp->grpmask;
+                rnp = rnp->parent;
+        } while (rnp != NULL);
+
+        /*
+         * We still hold the leaf rcu_node structure lock here, and
+         * irqs are still disabled.  The reason for this subterfuge is
+         * because invoking rcu_report_unblock_qs_rnp() with ->onofflock
+         * held leads to deadlock.
+         */
+        raw_spin_unlock(&rsp->onofflock); /* irqs remain disabled. */
+        rnp = rdp->mynode;
+        if (need_report & RCU_OFL_TASKS_NORM_GP)
+                rcu_report_unblock_qs_rnp(rnp, flags);
+        else
+                raw_spin_unlock_irqrestore(&rnp->lock, flags);
+        if (need_report & RCU_OFL_TASKS_EXP_GP)
+                rcu_report_exp_rnp(rsp, rnp, true);
 }
 
 #else /* #ifdef CONFIG_HOTPLUG_CPU */