- Reset hrtimers correctly when a CPU hotplug state traversal happens

"half-ways" and leaves hrtimers not (re-)initialized properly
 
 - Annotate accesses to a timer group's ignore flag to prevent KCSAN from
   raising data_race warnings
 
 - Make sure timer group initialization is visible to timer tree walkers and
   avoid a hypothetical race
 
 - Fix another race between CPU hotplug and idle entry/exit where timers on
   a fully idle system are getting ignored
 
 - Fix a case where an ignored signal is still being handled which it shouldn't
   be
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Merge tag 'timers_urgent_for_v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer fixes from Borislav Petkov:

 - Reset hrtimers correctly when a CPU hotplug state traversal happens
   "half-ways" and leaves hrtimers not (re-)initialized properly

 - Annotate accesses to a timer group's ignore flag to prevent KCSAN
   from raising data_race warnings

 - Make sure timer group initialization is visible to timer tree walkers
   and avoid a hypothetical race

 - Fix another race between CPU hotplug and idle entry/exit where timers
   on a fully idle system are getting ignored

 - Fix a case where an ignored signal is still being handled which it
   shouldn't be

* tag 'timers_urgent_for_v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  hrtimers: Handle CPU state correctly on hotplug
  timers/migration: Annotate accesses to ignore flag
  timers/migration: Enforce group initialization visibility to tree walkers
  timers/migration: Fix another race between hotplug and idle entry/exit
  signal/posixtimers: Handle ignore/blocked sequences correctly

include/linux/hrtimer.h

@@ -386,6 +386,7 @@ extern void __init hrtimers_init(void);
 extern void sysrq_timer_list_show(void);
 
 int hrtimers_prepare_cpu(unsigned int cpu);
+int hrtimers_cpu_starting(unsigned int cpu);
 #ifdef CONFIG_HOTPLUG_CPU
 int hrtimers_cpu_dying(unsigned int cpu);
 #else

kernel/cpu.c

@@ -2179,7 +2179,7 @@ static struct cpuhp_step cpuhp_hp_states[] = {
 	},
 	[CPUHP_AP_HRTIMERS_DYING] = {
 		.name			= "hrtimers:dying",
-		.startup.single		= NULL,
+		.startup.single		= hrtimers_cpu_starting,
 		.teardown.single	= hrtimers_cpu_dying,
 	},
 	[CPUHP_AP_TICK_DYING] = {

kernel/signal.c

@@ -2007,11 +2007,22 @@ void posixtimer_send_sigqueue(struct k_itimer *tmr)
 
 		if (!list_empty(&q->list)) {
 			/*
-			 * If task group is exiting with the signal already pending,
-			 * wait for __exit_signal() to do its job. Otherwise if
-			 * ignored, it's not supposed to be queued. Try to survive.
+			 * The signal was ignored and blocked. The timer
+			 * expiry queued it because blocked signals are
+			 * queued independent of the ignored state.
+			 *
+			 * The unblocking set SIGPENDING, but the signal
+			 * was not yet dequeued from the pending list.
+			 * So prepare_signal() sees unblocked and ignored,
+			 * which ends up here. Leave it queued like a
+			 * regular signal.
+			 *
+			 * The same happens when the task group is exiting
+			 * and the signal is already queued.
+			 * prepare_signal() treats SIGNAL_GROUP_EXIT as
+			 * ignored independent of its queued state. This
+			 * gets cleaned up in __exit_signal().
 			 */
-			WARN_ON_ONCE(!(t->signal->flags & SIGNAL_GROUP_EXIT));
 			goto out;
 		}
 
@@ -2046,17 +2057,25 @@ void posixtimer_send_sigqueue(struct k_itimer *tmr)
 		goto out;
 	}
 
-	/* This should never happen and leaks a reference count */
-	if (WARN_ON_ONCE(!hlist_unhashed(&tmr->ignored_list)))
-		hlist_del_init(&tmr->ignored_list);
-
 	if (unlikely(!list_empty(&q->list))) {
 		/* This holds a reference count already */
 		result = TRACE_SIGNAL_ALREADY_PENDING;
 		goto out;
 	}
 
-	posixtimer_sigqueue_getref(q);
+	/*
+	 * If the signal is on the ignore list, it got blocked after it was
+	 * ignored earlier. But nothing lifted the ignore. Move it back to
+	 * the pending list to be consistent with the regular signal
+	 * handling. This already holds a reference count.
+	 *
+	 * If it's not on the ignore list acquire a reference count.
+	 */
+	if (likely(hlist_unhashed(&tmr->ignored_list)))
+		posixtimer_sigqueue_getref(q);
+	else
+		hlist_del_init(&tmr->ignored_list);
+
 	posixtimer_queue_sigqueue(q, t, tmr->it_pid_type);
 	result = TRACE_SIGNAL_DELIVERED;
 out:

kernel/time/hrtimer.c

@@ -2202,6 +2202,15 @@ int hrtimers_prepare_cpu(unsigned int cpu)
 	}
 
 	cpu_base->cpu = cpu;
+	hrtimer_cpu_base_init_expiry_lock(cpu_base);
+	return 0;
+}
+
+int hrtimers_cpu_starting(unsigned int cpu)
+{
+	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
+
+	/* Clear out any left over state from a CPU down operation */
 	cpu_base->active_bases = 0;
 	cpu_base->hres_active = 0;
 	cpu_base->hang_detected = 0;
@@ -2210,7 +2219,6 @@ int hrtimers_prepare_cpu(unsigned int cpu)
 	cpu_base->expires_next = KTIME_MAX;
 	cpu_base->softirq_expires_next = KTIME_MAX;
 	cpu_base->online = 1;
-	hrtimer_cpu_base_init_expiry_lock(cpu_base);
 	return 0;
 }
 
@@ -2286,5 +2294,6 @@ int hrtimers_cpu_dying(unsigned int dying_cpu)
 void __init hrtimers_init(void)
 {
 	hrtimers_prepare_cpu(smp_processor_id());
+	hrtimers_cpu_starting(smp_processor_id());
 	open_softirq(HRTIMER_SOFTIRQ, hrtimer_run_softirq);
 }

kernel/time/timer_migration.c

@@ -534,8 +534,13 @@ static void __walk_groups(up_f up, struct tmigr_walk *data,
 			break;
 
 		child = group;
-		group = group->parent;
+		/*
+		 * Pairs with the store release on group connection
+		 * to make sure group initialization is visible.
+		 */
+		group = READ_ONCE(group->parent);
 		data->childmask = child->groupmask;
+		WARN_ON_ONCE(!data->childmask);
 	} while (group);
 }
 
@@ -564,7 +569,7 @@ static struct tmigr_event *tmigr_next_groupevt(struct tmigr_group *group)
 	while ((node = timerqueue_getnext(&group->events))) {
 		evt = container_of(node, struct tmigr_event, nextevt);
 
-		if (!evt->ignore) {
+		if (!READ_ONCE(evt->ignore)) {
 			WRITE_ONCE(group->next_expiry, evt->nextevt.expires);
 			return evt;
 		}
@@ -660,7 +665,7 @@ static bool tmigr_active_up(struct tmigr_group *group,
 	 * lock is held while updating the ignore flag in idle path. So this
 	 * state change will not be lost.
 	 */
-	group->groupevt.ignore = true;
+	WRITE_ONCE(group->groupevt.ignore, true);
 
 	return walk_done;
 }
@@ -721,6 +726,7 @@ bool tmigr_update_events(struct tmigr_group *group, struct tmigr_group *child,
 	union tmigr_state childstate, groupstate;
 	bool remote = data->remote;
 	bool walk_done = false;
+	bool ignore;
 	u64 nextexp;
 
 	if (child) {
@@ -739,11 +745,19 @@ bool tmigr_update_events(struct tmigr_group *group, struct tmigr_group *child,
 		nextexp = child->next_expiry;
 		evt = &child->groupevt;
 
-		evt->ignore = (nextexp == KTIME_MAX) ? true : false;
+		/*
+		 * This can race with concurrent idle exit (activate).
+		 * If the current writer wins, a useless remote expiration may
+		 * be scheduled. If the activate wins, the event is properly
+		 * ignored.
+		 */
+		ignore = (nextexp == KTIME_MAX) ? true : false;
+		WRITE_ONCE(evt->ignore, ignore);
 	} else {
 		nextexp = data->nextexp;
 
 		first_childevt = evt = data->evt;
+		ignore = evt->ignore;
 
 		/*
 		 * Walking the hierarchy is required in any case when a
@@ -769,7 +783,7 @@ bool tmigr_update_events(struct tmigr_group *group, struct tmigr_group *child,
 		 * first event information of the group is updated properly and
 		 * also handled properly, so skip this fast return path.
 		 */
-		if (evt->ignore && !remote && group->parent)
+		if (ignore && !remote && group->parent)
 			return true;
 
 		raw_spin_lock(&group->lock);
@@ -783,7 +797,7 @@ bool tmigr_update_events(struct tmigr_group *group, struct tmigr_group *child,
 	 * queue when the expiry time changed only or when it could be ignored.
 	 */
 	if (timerqueue_node_queued(&evt->nextevt)) {
-		if ((evt->nextevt.expires == nextexp) && !evt->ignore) {
+		if ((evt->nextevt.expires == nextexp) && !ignore) {
 			/* Make sure not to miss a new CPU event with the same expiry */
 			evt->cpu = first_childevt->cpu;
 			goto check_toplvl;
@@ -793,7 +807,7 @@ bool tmigr_update_events(struct tmigr_group *group, struct tmigr_group *child,
 			WRITE_ONCE(group->next_expiry, KTIME_MAX);
 	}
 
-	if (evt->ignore) {
+	if (ignore) {
 		/*
 		 * When the next child event could be ignored (nextexp is
 		 * KTIME_MAX) and there was no remote timer handling before or
@@ -1487,6 +1501,21 @@ static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
 	s.seq = 0;
 	atomic_set(&group->migr_state, s.state);
 
+	/*
+	 * If this is a new top-level, prepare its groupmask in advance.
+	 * This avoids accidents where yet another new top-level is
+	 * created in the future and made visible before the current groupmask.
+	 */
+	if (list_empty(&tmigr_level_list[lvl])) {
+		group->groupmask = BIT(0);
+		/*
+		 * The previous top level has prepared its groupmask already,
+		 * simply account it as the first child.
+		 */
+		if (lvl > 0)
+			group->num_children = 1;
+	}
+
 	timerqueue_init_head(&group->events);
 	timerqueue_init(&group->groupevt.nextevt);
 	group->groupevt.nextevt.expires = KTIME_MAX;
@@ -1550,8 +1579,25 @@ static void tmigr_connect_child_parent(struct tmigr_group *child,
 	raw_spin_lock_irq(&child->lock);
 	raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING);
 
-	child->parent = parent;
-	child->groupmask = BIT(parent->num_children++);
+	if (activate) {
+		/*
+		 * @child is the old top and @parent the new one. In this
+		 * case groupmask is pre-initialized and @child already
+		 * accounted, along with its new sibling corresponding to the
+		 * CPU going up.
+		 */
+		WARN_ON_ONCE(child->groupmask != BIT(0) || parent->num_children != 2);
+	} else {
+		/* Adding @child for the CPU going up to @parent. */
+		child->groupmask = BIT(parent->num_children++);
+	}
+
+	/*
+	 * Make sure parent initialization is visible before publishing it to a
+	 * racing CPU entering/exiting idle. This RELEASE barrier enforces an
+	 * address dependency that pairs with the READ_ONCE() in __walk_groups().
+	 */
+	smp_store_release(&child->parent, parent);
 
 	raw_spin_unlock(&parent->lock);
 	raw_spin_unlock_irq(&child->lock);