fsnotify: generic notification queue and waitq

inotify needs to do asyc notification in which event information is stored
on a queue until the listener is ready to receive it.  This patch
implements a generic notification queue for inotify (and later fanotify) to
store events to be sent at a later time.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>

3 files changed, 241 insertions, 7 deletions

diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
index 83b8ec0a8ec2..4dc240824b2d 100644
--- a/fs/notify/fsnotify.h
+++ b/fs/notify/fsnotify.h
@@ -13,8 +13,12 @@ extern struct list_head fsnotify_groups;
 /* all bitwise OR of all event types (FS_*) for all fsnotify_groups */
 extern __u32 fsnotify_mask;
 
+/* destroy all events sitting in this groups notification queue */
+extern void fsnotify_flush_notify(struct fsnotify_group *group);
+
 /* final kfree of a group */
 extern void fsnotify_final_destroy_group(struct fsnotify_group *group);
+
 /* run the list of all marks associated with inode and flag them to be freed */
 extern void fsnotify_clear_marks_by_inode(struct inode *inode);
 /*
@@ -22,4 +26,9 @@ extern void fsnotify_clear_marks_by_inode(struct inode *inode);
  * about events that happen to its children.
  */
 extern void __fsnotify_update_child_dentry_flags(struct inode *inode);
+
+/* allocate and destroy and event holder to attach events to notification/access queues */
+extern struct fsnotify_event_holder *fsnotify_alloc_event_holder(void);
+extern void fsnotify_destroy_event_holder(struct fsnotify_event_holder *holder);
+
 #endif  /* __FS_NOTIFY_FSNOTIFY_H_ */
diff --git a/fs/notify/group.c b/fs/notify/group.c
index a29d2fa67927..0e1677144bc5 100644
--- a/fs/notify/group.c
+++ b/fs/notify/group.c
@@ -91,6 +91,9 @@ static void fsnotify_get_group(struct fsnotify_group *group)
  */
 void fsnotify_final_destroy_group(struct fsnotify_group *group)
 {
+        /* clear the notification queue of all events */
+        fsnotify_flush_notify(group);
+
         if (group->ops->free_group_priv)
                 group->ops->free_group_priv(group);
 
@@ -214,6 +217,12 @@ struct fsnotify_group *fsnotify_obtain_group(unsigned int group_num, __u32 mask,
         group->group_num = group_num;
         group->mask = mask;
 
+        mutex_init(&group->notification_mutex);
+        INIT_LIST_HEAD(&group->notification_list);
+        init_waitqueue_head(&group->notification_waitq);
+        group->q_len = 0;
+        group->max_events = UINT_MAX;
+
         spin_lock_init(&group->mark_lock);
         atomic_set(&group->num_marks, 0);
         INIT_LIST_HEAD(&group->mark_entries);
diff --git a/fs/notify/notification.c b/fs/notify/notification.c
index b8e9a87f8f58..dddecc74e63d 100644
--- a/fs/notify/notification.c
+++ b/fs/notify/notification.c
@@ -16,6 +16,21 @@
  *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+/*
+ * Basic idea behind the notification queue: An fsnotify group (like inotify)
+ * sends the userspace notification about events asyncronously some time after
+ * the event happened.  When inotify gets an event it will need to add that
+ * event to the group notify queue.  Since a single event might need to be on
+ * multiple group's notification queues we can't add the event directly to each
+ * queue and instead add a small "event_holder" to each queue.  This event_holder
+ * has a pointer back to the original event.  Since the majority of events are
+ * going to end up on one, and only one, notification queue we embed one
+ * event_holder into each event.  This means we have a single allocation instead
+ * of always needing two.  If the embedded event_holder is already in use by
+ * another group a new event_holder (from fsnotify_event_holder_cachep) will be
+ * allocated and used.
+ */
+
 #include <linux/fs.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
@@ -33,6 +48,21 @@
 #include "fsnotify.h"
 
 static struct kmem_cache *fsnotify_event_cachep;
+static struct kmem_cache *fsnotify_event_holder_cachep;
+/*
+ * This is a magic event we send when the q is too full.  Since it doesn't
+ * hold real event information we just keep one system wide and use it any time
+ * it is needed.  It's refcnt is set 1 at kernel init time and will never
+ * get set to 0 so it will never get 'freed'
+ */
+static struct fsnotify_event q_overflow_event;
+
+/* return true if the notify queue is empty, false otherwise */
+bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
+{
+        BUG_ON(!mutex_is_locked(&group->notification_mutex));
+        return list_empty(&group->notification_list) ? true : false;
+}
 
 void fsnotify_get_event(struct fsnotify_event *event)
 {
@@ -52,19 +82,176 @@ void fsnotify_put_event(struct fsnotify_event *event)
         }
 }
 
+struct fsnotify_event_holder *fsnotify_alloc_event_holder(void)
+{
+        return kmem_cache_alloc(fsnotify_event_holder_cachep, GFP_KERNEL);
+}
+
+void fsnotify_destroy_event_holder(struct fsnotify_event_holder *holder)
+{
+        kmem_cache_free(fsnotify_event_holder_cachep, holder);
+}
+
+/*
+ * check if 2 events contain the same information.
+ */
+static bool event_compare(struct fsnotify_event *old, struct fsnotify_event *new)
+{
+        if ((old->mask == new->mask) &&
+            (old->to_tell == new->to_tell) &&
+            (old->data_type == new->data_type)) {
+                switch (old->data_type) {
+                case (FSNOTIFY_EVENT_INODE):
+                        if (old->inode == new->inode)
+                                return true;
+                        break;
+                case (FSNOTIFY_EVENT_PATH):
+                        if ((old->path.mnt == new->path.mnt) &&
+                            (old->path.dentry == new->path.dentry))
+                                return true;
+                case (FSNOTIFY_EVENT_NONE):
+                        return true;
+                };
+        }
+        return false;
+}
+
 /*
- * Allocate a new event which will be sent to each group's handle_event function
- * if the group was interested in this particular event.
+ * Add an event to the group notification queue.  The group can later pull this
+ * event off the queue to deal with.  If the event is successfully added to the
+ * group's notification queue, a reference is taken on event.
  */
-struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32 mask,
-                                             void *data, int data_type)
+int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event)
+{
+        struct fsnotify_event_holder *holder = NULL;
+        struct list_head *list = &group->notification_list;
+        struct fsnotify_event_holder *last_holder;
+        struct fsnotify_event *last_event;
+
+        /*
+         * There is one fsnotify_event_holder embedded inside each fsnotify_event.
+         * Check if we expect to be able to use that holder.  If not alloc a new
+         * holder.
+         * For the overflow event it's possible that something will use the in
+         * event holder before we get the lock so we may need to jump back and
+         * alloc a new holder, this can't happen for most events...
+         */
+        if (!list_empty(&event->holder.event_list)) {
+alloc_holder:
+                holder = fsnotify_alloc_event_holder();
+                if (!holder)
+                        return -ENOMEM;
+        }
+
+        mutex_lock(&group->notification_mutex);
+
+        if (group->q_len >= group->max_events)
+                event = &q_overflow_event;
+
+        spin_lock(&event->lock);
+
+        if (list_empty(&event->holder.event_list)) {
+                if (unlikely(holder))
+                        fsnotify_destroy_event_holder(holder);
+                holder = &event->holder;
+        } else if (unlikely(!holder)) {
+                /* between the time we checked above and got the lock the in
+                 * event holder was used, go back and get a new one */
+                spin_unlock(&event->lock);
+                mutex_unlock(&group->notification_mutex);
+                goto alloc_holder;
+        }
+
+        if (!list_empty(list)) {
+                last_holder = list_entry(list->prev, struct fsnotify_event_holder, event_list);
+                last_event = last_holder->event;
+                if (event_compare(last_event, event)) {
+                        spin_unlock(&event->lock);
+                        mutex_unlock(&group->notification_mutex);
+                        if (holder != &event->holder)
+                                fsnotify_destroy_event_holder(holder);
+                        return 0;
+                }
+        }
+
+        group->q_len++;
+        holder->event = event;
+
+        fsnotify_get_event(event);
+        list_add_tail(&holder->event_list, list);
+        spin_unlock(&event->lock);
+        mutex_unlock(&group->notification_mutex);
+
+        wake_up(&group->notification_waitq);
+        return 0;
+}
+
+/*
+ * Remove and return the first event from the notification list.  There is a
+ * reference held on this event since it was on the list.  It is the responsibility
+ * of the caller to drop this reference.
+ */
+struct fsnotify_event *fsnotify_remove_notify_event(struct fsnotify_group *group)
 {
         struct fsnotify_event *event;
+        struct fsnotify_event_holder *holder;
 
-        event = kmem_cache_alloc(fsnotify_event_cachep, GFP_KERNEL);
-        if (!event)
-                return NULL;
+        BUG_ON(!mutex_is_locked(&group->notification_mutex));
 
+        holder = list_first_entry(&group->notification_list, struct fsnotify_event_holder, event_list);
+
+        event = holder->event;
+
+        spin_lock(&event->lock);
+        holder->event = NULL;
+        list_del_init(&holder->event_list);
+        spin_unlock(&event->lock);
+
+        /* event == holder means we are referenced through the in event holder */
+        if (holder != &event->holder)
+                fsnotify_destroy_event_holder(holder);
+
+        group->q_len--;
+
+        return event;
+}
+
+/*
+ * This will not remove the event, that must be done with fsnotify_remove_notify_event()
+ */
+struct fsnotify_event *fsnotify_peek_notify_event(struct fsnotify_group *group)
+{
+        struct fsnotify_event *event;
+        struct fsnotify_event_holder *holder;
+
+        BUG_ON(!mutex_is_locked(&group->notification_mutex));
+
+        holder = list_first_entry(&group->notification_list, struct fsnotify_event_holder, event_list);
+        event = holder->event;
+
+        return event;
+}
+
+/*
+ * Called when a group is being torn down to clean up any outstanding
+ * event notifications.
+ */
+void fsnotify_flush_notify(struct fsnotify_group *group)
+{
+        struct fsnotify_event *event;
+
+        mutex_lock(&group->notification_mutex);
+        while (!fsnotify_notify_queue_is_empty(group)) {
+                event = fsnotify_remove_notify_event(group);
+                fsnotify_put_event(event); /* matches fsnotify_add_notify_event */
+        }
+        mutex_unlock(&group->notification_mutex);
+}
+
+static void initialize_event(struct fsnotify_event *event)
+{
+        event->holder.event = NULL;
+        INIT_LIST_HEAD(&event->holder.event_list);
         atomic_set(&event->refcnt, 1);
 
         spin_lock_init(&event->lock);
@@ -72,7 +259,32 @@ struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32 mask,
         event->path.dentry = NULL;
         event->path.mnt = NULL;
         event->inode = NULL;
+        event->data_type = FSNOTIFY_EVENT_NONE;
 
+        event->to_tell = NULL;
+}
+
+/*
+ * fsnotify_create_event - Allocate a new event which will be sent to each
+ * group's handle_event function if the group was interested in this
+ * particular event.
+ *
+ * @to_tell the inode which is supposed to receive the event (sometimes a
+ *      parent of the inode to which the event happened.
+ * @mask what actually happened.
+ * @data pointer to the object which was actually affected
+ * @data_type flag indication if the data is a file, path, inode, nothing...
+ */
+struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32 mask,
+                                             void *data, int data_type)
+{
+        struct fsnotify_event *event;
+
+        event = kmem_cache_alloc(fsnotify_event_cachep, GFP_KERNEL);
+        if (!event)
+                return NULL;
+
+        initialize_event(event);
         event->to_tell = to_tell;
 
         switch (data_type) {
@@ -114,6 +326,10 @@ struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32 mask,
 __init int fsnotify_notification_init(void)
 {
         fsnotify_event_cachep = KMEM_CACHE(fsnotify_event, SLAB_PANIC);
+        fsnotify_event_holder_cachep = KMEM_CACHE(fsnotify_event_holder, SLAB_PANIC);
+
+        initialize_event(&q_overflow_event);
+        q_overflow_event.mask = FS_Q_OVERFLOW;
 
         return 0;
 }