[PATCH 2/6] xenbus: implement the xenwatch multithreading framework

[Dongli Zhang]

This is the 2nd patch of a (6-patch) patch set.

This patch implements the xenwatch multithreading framework to create or
destroy the per-domU xenwatch thread. The xenwatch thread is created or
destroyed during xenbus device probing or removing (that is,
xenbus_dev_probe() or xenbus_dev_remove()) if the corresponding pv driver
has xenwatch multithreading feature enabled. As there is only one single
per-domU xenwatch thread for each domid, probing the xenbus device for the
same domid again would not create the thread for the same domid again, but
only increment the reference count of the thread's mtwatch domain. When a
xenbus device is removed, the reference count is decremented. The per-domU
xenwatch thread is destroyed when the reference count of its mtwatch domain
is zero, that is, al xenbus devices (whose mtwatch feature is enabled) of
such mtwatch domain are removed.

Therefore, a domid has its own per-domU xenwatch thread only when it is
attached with dom0 backend xenbus device whose pv driver has the feature
enabled. The domid would not have its own xenwatch thread when it is not
running any mtwatch-enabled xenbus device.

When a watch (with xenwatch multithreading enabled) is unregistered, we
will generally traverse all mtwatch domains to remove all inflight pending
events fired by such watch. However, one optimization in this patch is we
only need to remove pending events from a specific mtwatch domain when the
watch is registered for a specific domid, that is, when its owner_id field
is non-zero.

Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
---
 drivers/xen/xenbus/xenbus_probe.c |   6 +
 drivers/xen/xenbus/xenbus_xs.c    | 273 ++++++++++++++++++++++++++++++++++++++
 include/xen/xenbus.h              |   3 +
 3 files changed, 282 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 5b47188..5755596 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -236,6 +236,9 @@ int xenbus_dev_probe(struct device *_dev)
 	if (err)
 		goto fail;
 
+	if (xen_mtwatch && drv->use_mtwatch)
+		mtwatch_create_domain(dev->otherend_id);
+
 	err = watch_otherend(dev);
 	if (err) {
 		dev_warn(&dev->dev, "watch_otherend on %s failed.\n",
@@ -263,6 +266,9 @@ int xenbus_dev_remove(struct device *_dev)
 	if (drv->remove)
 		drv->remove(dev);
 
+	if (xen_mtwatch && drv->use_mtwatch)
+		mtwatch_put_domain(dev->otherend_id);
+
 	free_otherend_details(dev);
 
 	xenbus_switch_state(dev, XenbusStateClosed);
diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index 3f137d2..741dc54 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -108,6 +108,201 @@ static __init int xen_parse_mtwatch(char *arg)
 }
 early_param("xen_mtwatch", xen_parse_mtwatch);
 
+struct mtwatch_domain *mtwatch_find_domain(domid_t domid)
+{
+	struct mtwatch_domain *domain;
+	int hash = MTWATCH_HASH(domid);
+	struct hlist_head *hash_head = &mtwatch_info->domain_hash[hash];
+
+	hlist_for_each_entry_rcu(domain, hash_head, hash_node) {
+		if (domain->domid == domid)
+			return domain;
+	}
+
+	return NULL;
+}
+
+/* per-domU thread for xenwatch multithreading. */
+static int mtwatch_thread(void *arg)
+{
+	struct mtwatch_domain *domain = (struct mtwatch_domain *) arg;
+	struct list_head *ent;
+	struct xs_watch_event *event;
+
+	domain->pid = current->pid;
+
+	for (;;) {
+		wait_event_interruptible(domain->events_wq,
+					 !list_empty(&domain->events) ||
+					 domain->state == MTWATCH_DOMAIN_DOWN);
+
+		if (domain->state == MTWATCH_DOMAIN_DOWN &&
+		    list_empty(&domain->events))
+			break;
+
+		mutex_lock(&domain->domain_mutex);
+
+		spin_lock(&domain->events_lock);
+		ent = domain->events.next;
+		if (ent != &domain->events)
+			list_del(ent);
+		spin_unlock(&domain->events_lock);
+
+		if (ent != &domain->events) {
+			event = list_entry(ent, struct xs_watch_event, list);
+			event->handle->callback(event->handle, event->path,
+						event->token);
+			kfree(event);
+		}
+
+		mutex_unlock(&domain->domain_mutex);
+	}
+
+	/*
+	 * domain->state is already set to MTWATCH_DOMAIN_DOWN (to avoid
+	 * new event to domain->events) when above for loop breaks, so
+	 * that there is no requirement to cleanup domain->events again.
+	 */
+
+	spin_lock(&mtwatch_info->domain_lock);
+	list_del_rcu(&domain->list_node);
+	spin_unlock(&mtwatch_info->domain_lock);
+
+	spin_lock(&mtwatch_info->purge_lock);
+	list_add(&domain->purge_node, &mtwatch_info->purge_list);
+	spin_unlock(&mtwatch_info->purge_lock);
+
+	schedule_work(&mtwatch_info->purge_work);
+
+	return 0;
+}
+
+static void delayed_destroy_domain(struct rcu_head *head)
+{
+	struct mtwatch_domain *domain;
+
+	domain = container_of(head, struct mtwatch_domain, rcu);
+	kfree(domain);
+}
+
+static void xen_mtwatch_purge_domain(struct work_struct *work)
+{
+	struct mtwatch_domain *domain;
+	struct list_head *node;
+
+	while (!list_empty(&mtwatch_info->purge_list)) {
+
+		spin_lock(&mtwatch_info->purge_lock);
+		node = mtwatch_info->purge_list.next;
+		if (node != &mtwatch_info->purge_list)
+			list_del(node);
+		spin_unlock(&mtwatch_info->purge_lock);
+
+		if (node != &mtwatch_info->purge_list) {
+			domain = list_entry(node, struct mtwatch_domain,
+					    purge_node);
+			kthread_stop(domain->task);
+
+			call_rcu(&domain->rcu, delayed_destroy_domain);
+		}
+	}
+}
+
+/* Running in the context of default xenwatch kthread. */
+void mtwatch_create_domain(domid_t domid)
+{
+	struct mtwatch_domain *domain;
+
+	if (!domid) {
+		pr_err("Default xenwatch thread is for dom0\n");
+		return;
+	}
+
+	spin_lock(&mtwatch_info->domain_lock);
+
+	domain = mtwatch_find_domain(domid);
+	if (domain) {
+		atomic_inc(&domain->refcnt);
+		spin_unlock(&mtwatch_info->domain_lock);
+		return;
+	}
+
+	domain = kzalloc(sizeof(*domain), GFP_ATOMIC);
+	if (!domain) {
+		spin_unlock(&mtwatch_info->domain_lock);
+		pr_err("Failed to allocate memory for mtwatch thread %d\n",
+		       domid);
+		return;
+	}
+
+	domain->domid = domid;
+	atomic_set(&domain->refcnt, 1);
+	mutex_init(&domain->domain_mutex);
+	INIT_LIST_HEAD(&domain->purge_node);
+
+	init_waitqueue_head(&domain->events_wq);
+	spin_lock_init(&domain->events_lock);
+	INIT_LIST_HEAD(&domain->events);
+
+	list_add_tail_rcu(&domain->list_node, &mtwatch_info->domain_list);
+
+	hlist_add_head_rcu(&domain->hash_node,
+			   &mtwatch_info->domain_hash[MTWATCH_HASH(domid)]);
+
+	spin_unlock(&mtwatch_info->domain_lock);
+
+	domain->task = kthread_run(mtwatch_thread, domain,
+				   "xen-mtwatch-%d", domid);
+
+	if (!domain->task) {
+		pr_err("mtwatch kthread creation is failed\n");
+		domain->state = MTWATCH_DOMAIN_DOWN;
+
+		return;
+	}
+
+	domain->state = MTWATCH_DOMAIN_UP;
+}
+
+/* Running in the context of default xenwatch kthread. */
+void mtwatch_put_domain(domid_t domid)
+{
+	struct mtwatch_domain *domain;
+
+	spin_lock(&mtwatch_info->domain_lock);
+
+	domain = mtwatch_find_domain(domid);
+	if (!domain) {
+		spin_unlock(&mtwatch_info->domain_lock);
+		pr_err("mtwatch kthread for domid=%d does not exist\n",
+		       domid);
+		return;
+	}
+
+	if (atomic_dec_and_test(&domain->refcnt)) {
+
+		hlist_del_rcu(&domain->hash_node);
+
+		if (!domain->task) {
+			/*
+			 * As the task is failed to initialize during
+			 * mtwatch_create_domain(), we do not need to wait
+			 * for the kernel thread to complete.
+			 */
+			list_del_rcu(&domain->list_node);
+			call_rcu(&domain->rcu, delayed_destroy_domain);
+		} else {
+			spin_lock(&domain->events_lock);
+			domain->state = MTWATCH_DOMAIN_DOWN;
+			spin_unlock(&domain->events_lock);
+
+			wake_up(&domain->events_wq);
+		}
+	}
+
+	spin_unlock(&mtwatch_info->domain_lock);
+}
+
 static void xs_suspend_enter(void)
 {
 	spin_lock(&xs_state_lock);
@@ -793,6 +988,80 @@ int register_xenbus_watch(struct xenbus_watch *watch)
 }
 EXPORT_SYMBOL_GPL(register_xenbus_watch);
 
+static void __unregister_single_mtwatch(struct xenbus_watch *watch,
+					struct mtwatch_domain *domain)
+{
+	struct xs_watch_event *event, *tmp;
+
+	if (current->pid != domain->pid)
+		mutex_lock(&domain->domain_mutex);
+
+	spin_lock(&domain->events_lock);
+	list_for_each_entry_safe(event, tmp,
+				 &domain->events, list) {
+		if (event->handle != watch)
+			continue;
+		list_del(&event->list);
+		kfree(event);
+	}
+	spin_unlock(&domain->events_lock);
+
+	if (current->pid != domain->pid)
+		mutex_unlock(&domain->domain_mutex);
+}
+
+static void unregister_single_mtwatch(struct xenbus_watch *watch,
+				      domid_t domid)
+{
+	struct mtwatch_domain *domain;
+	bool found = false;
+
+	rcu_read_lock();
+
+	list_for_each_entry_rcu(domain, &mtwatch_info->domain_list,
+				list_node) {
+		if (domain->domid == domid) {
+			found = true;
+			__unregister_single_mtwatch(watch, domain);
+		}
+	}
+
+	WARN_ON_ONCE(unlikely(!found));
+
+	rcu_read_unlock();
+}
+
+static void unregister_all_mtwatch(struct xenbus_watch *watch)
+{
+	struct mtwatch_domain *domain;
+
+	rcu_read_lock();
+
+	list_for_each_entry_rcu(domain, &mtwatch_info->domain_list,
+				list_node) {
+		__unregister_single_mtwatch(watch, domain);
+	}
+
+	rcu_read_unlock();
+}
+
+static void unregister_mtwatch(struct xenbus_watch *watch)
+{
+	/*
+	 * Generally, to unregister a watch. we need to traverse all
+	 * mtwatch domains to remove all inflight pending watch events for
+	 * such watch.
+	 *
+	 * One exception is we only need to remove pending watch events
+	 * from a single mtwatch domain when the watch is registered for a
+	 * specific domid.
+	 */
+	if (watch->owner_id)
+		unregister_single_mtwatch(watch, watch->owner_id);
+	else
+		unregister_all_mtwatch(watch);
+}
+
 void unregister_xenbus_watch(struct xenbus_watch *watch)
 {
 	struct xs_watch_event *event, *tmp;
@@ -831,6 +1100,9 @@ void unregister_xenbus_watch(struct xenbus_watch *watch)
 
 	if (current->pid != xenwatch_pid)
 		mutex_unlock(&xenwatch_mutex);
+
+	if (xen_mtwatch && watch->get_domid)
+		unregister_mtwatch(watch);
 }
 EXPORT_SYMBOL_GPL(unregister_xenbus_watch);
 
@@ -954,6 +1226,7 @@ int xs_init(void)
 
 		spin_lock_init(&mtwatch_info->purge_lock);
 		INIT_LIST_HEAD(&mtwatch_info->purge_list);
+		INIT_WORK(&mtwatch_info->purge_work, xen_mtwatch_purge_domain);
 
 		xen_mtwatch = true;
 
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index e807114..4ac2cee 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -241,6 +241,9 @@ extern const struct file_operations xen_xenbus_fops;
 extern struct xenstore_domain_interface *xen_store_interface;
 extern int xen_store_evtchn;
 
+void mtwatch_create_domain(domid_t domid);
+void mtwatch_put_domain(domid_t domid);
+
 extern bool xen_mtwatch;
 
 #define MTWATCH_HASH_SIZE 256
-- 
2.7.4