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SubjectRe: [PATCH 2/6] xenbus: implement the xenwatch multithreading framework
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On 14/09/18 09:34, Dongli Zhang wrote:
> 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@oracle.com>
> ---
> 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);
> + }

This is nearly the same coding as xenwatch_thread(). Why don't you
define a new (sub-)structure containing the needed elements and move
xenwatch_pid, watch_events_waitq, watch_events, xenwatch_mutex,
watch_events_lock into such a structure? Then you could a common
function for both purposes (you'd need to set state for
xenwatch_thread() to DOMAIN_UP and add a callback for testing the
thread end condition).

> +
> + /*
> + * 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");

Should we really exclude dom0? What if a driver domain wants to support
a dom0 based frontend?

> + 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);

No alloc error messages, please!

> + 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)

Again: 0 as a special value isn't a good idea. Maybe use one of the
reserved DOMIDs, like DOMID_SELF?


Juergen

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