Merge tag 'rcu-fixes-v6.7' of git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks

Pull RCU fixes from Frederic Weisbecker:

 - Fix a lock inversion between scheduler and RCU introduced in
   v6.2-rc4. The scenario could trigger on any user of RCU_NOCB
   (mostly Android but also nohz_full)

 - Fix PF_IDLE semantic changes introduced in v6.6-rc3 breaking
   some RCU-Tasks and RCU-Tasks-Trace expectations as to what
   exactly is an idle task. This resulted in potential spurious
   stalls and warnings.

* tag 'rcu-fixes-v6.7' of git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks:
  rcu/tasks-trace: Handle new PF_IDLE semantics
  rcu/tasks: Handle new PF_IDLE semantics
  rcu: Introduce rcu_cpu_online()
  rcu: Break rcu_node_0 --> &rq->__lock order

Author: torvalds

kernel/rcu/rcu.h

@@ -500,6 +500,7 @@ static inline void rcu_expedite_gp(void) { }
 static inline void rcu_unexpedite_gp(void) { }
 static inline void rcu_async_hurry(void) { }
 static inline void rcu_async_relax(void) { }
+static inline bool rcu_cpu_online(int cpu) { return true; }
 #else /* #ifdef CONFIG_TINY_RCU */
 bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
@@ -509,6 +510,7 @@ void rcu_unexpedite_gp(void);
 void rcu_async_hurry(void);
 void rcu_async_relax(void);
 void rcupdate_announce_bootup_oddness(void);
+bool rcu_cpu_online(int cpu);
 #ifdef CONFIG_TASKS_RCU_GENERIC
 void show_rcu_tasks_gp_kthreads(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */

kernel/rcu/tasks.h

@@ -895,10 +895,36 @@ static void rcu_tasks_pregp_step(struct list_head *hop)
 	synchronize_rcu();
 }
 
+/* Check for quiescent states since the pregp's synchronize_rcu() */
+static bool rcu_tasks_is_holdout(struct task_struct *t)
+{
+	int cpu;
+
+	/* Has the task been seen voluntarily sleeping? */
+	if (!READ_ONCE(t->on_rq))
+		return false;
+
+	/*
+	 * Idle tasks (or idle injection) within the idle loop are RCU-tasks
+	 * quiescent states. But CPU boot code performed by the idle task
+	 * isn't a quiescent state.
+	 */
+	if (is_idle_task(t))
+		return false;
+
+	cpu = task_cpu(t);
+
+	/* Idle tasks on offline CPUs are RCU-tasks quiescent states. */
+	if (t == idle_task(cpu) && !rcu_cpu_online(cpu))
+		return false;
+
+	return true;
+}
+
 /* Per-task initial processing. */
 static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
 {
-	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
+	if (t != current && rcu_tasks_is_holdout(t)) {
 		get_task_struct(t);
 		t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
 		WRITE_ONCE(t->rcu_tasks_holdout, true);
@@ -947,7 +973,7 @@ static void check_holdout_task(struct task_struct *t,
 
 	if (!READ_ONCE(t->rcu_tasks_holdout) ||
 	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
-	    !READ_ONCE(t->on_rq) ||
+	    !rcu_tasks_is_holdout(t) ||
 	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
 	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
 		WRITE_ONCE(t->rcu_tasks_holdout, false);
@@ -1525,7 +1551,7 @@ static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
 	} else {
 		// The task is not running, so C-language access is safe.
 		nesting = t->trc_reader_nesting;
-		WARN_ON_ONCE(ofl && task_curr(t) && !is_idle_task(t));
+		WARN_ON_ONCE(ofl && task_curr(t) && (t != idle_task(task_cpu(t))));
 		if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && ofl)
 			n_heavy_reader_ofl_updates++;
 	}

kernel/rcu/tree.c

@@ -755,14 +755,19 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
 }
 
 /*
- * Return true if the specified CPU has passed through a quiescent
- * state by virtue of being in or having passed through an dynticks
- * idle state since the last call to dyntick_save_progress_counter()
- * for this same CPU, or by virtue of having been offline.
+ * Returns positive if the specified CPU has passed through a quiescent state
+ * by virtue of being in or having passed through an dynticks idle state since
+ * the last call to dyntick_save_progress_counter() for this same CPU, or by
+ * virtue of having been offline.
+ *
+ * Returns negative if the specified CPU needs a force resched.
+ *
+ * Returns zero otherwise.
  */
 static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 {
 	unsigned long jtsq;
+	int ret = 0;
 	struct rcu_node *rnp = rdp->mynode;
 
 	/*
@@ -848,8 +853,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	    (time_after(jiffies, READ_ONCE(rdp->last_fqs_resched) + jtsq * 3) ||
 	     rcu_state.cbovld)) {
 		WRITE_ONCE(rdp->rcu_urgent_qs, true);
-		resched_cpu(rdp->cpu);
 		WRITE_ONCE(rdp->last_fqs_resched, jiffies);
+		ret = -1;
 	}
 
 	/*
@@ -862,8 +867,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	if (time_after(jiffies, rcu_state.jiffies_resched)) {
 		if (time_after(jiffies,
 			       READ_ONCE(rdp->last_fqs_resched) + jtsq)) {
-			resched_cpu(rdp->cpu);
 			WRITE_ONCE(rdp->last_fqs_resched, jiffies);
+			ret = -1;
 		}
 		if (IS_ENABLED(CONFIG_IRQ_WORK) &&
 		    !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
@@ -892,7 +897,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 		}
 	}
 
-	return 0;
+	return ret;
 }
 
 /* Trace-event wrapper function for trace_rcu_future_grace_period.  */
@@ -2271,15 +2276,15 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
 {
 	int cpu;
 	unsigned long flags;
-	unsigned long mask;
-	struct rcu_data *rdp;
 	struct rcu_node *rnp;
 
 	rcu_state.cbovld = rcu_state.cbovldnext;
 	rcu_state.cbovldnext = false;
 	rcu_for_each_leaf_node(rnp) {
+		unsigned long mask = 0;
+		unsigned long rsmask = 0;
+
 		cond_resched_tasks_rcu_qs();
-		mask = 0;
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		rcu_state.cbovldnext |= !!rnp->cbovldmask;
 		if (rnp->qsmask == 0) {
@@ -2297,11 +2302,17 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
 			continue;
 		}
 		for_each_leaf_node_cpu_mask(rnp, cpu, rnp->qsmask) {
+			struct rcu_data *rdp;
+			int ret;
+
 			rdp = per_cpu_ptr(&rcu_data, cpu);
-			if (f(rdp)) {
+			ret = f(rdp);
+			if (ret > 0) {
 				mask |= rdp->grpmask;
 				rcu_disable_urgency_upon_qs(rdp);
 			}
+			if (ret < 0)
+				rsmask |= rdp->grpmask;
 		}
 		if (mask != 0) {
 			/* Idle/offline CPUs, report (releases rnp->lock). */
@@ -2310,6 +2321,9 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
 			/* Nothing to do here, so just drop the lock. */
 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		}
+
+		for_each_leaf_node_cpu_mask(rnp, cpu, rsmask)
+			resched_cpu(cpu);
 	}
 }
 
@@ -4195,6 +4209,13 @@ static bool rcu_rdp_cpu_online(struct rcu_data *rdp)
 	return !!(rdp->grpmask & rcu_rnp_online_cpus(rdp->mynode));
 }
 
+bool rcu_cpu_online(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+
+	return rcu_rdp_cpu_online(rdp);
+}
+
 #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
 
 /*