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Subject[PATCH 06/12] fsnotify: generic notification queue and waitq
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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>
---
fs/notify/fsnotify.h | 9 +
fs/notify/group.c | 9 +
fs/notify/notification.c | 230 +++++++++++++++++++++++++++++++++++++-
include/linux/fsnotify_backend.h | 37 ++++++
4 files changed, 278 insertions(+), 7 deletions(-)
diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
index 83b8ec0..4dc2408 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 a29d2fa..0e16771 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 b8e9a87..dddecc7 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;
}
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 9ea800e..15f8f82 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -119,6 +119,13 @@ struct fsnotify_group {

const struct fsnotify_ops *ops; /* how this group handles things */

+ /* needed to send notification to userspace */
+ struct mutex notification_mutex; /* protect the notification_list */
+ struct list_head notification_list; /* list of event_holder this group needs to send to userspace */
+ wait_queue_head_t notification_waitq; /* read() on the notification file blocks on this waitq */
+ unsigned int q_len; /* events on the queue */
+ unsigned int max_events; /* maximum events allowed on the list */
+
/* stores all fastapth entries assoc with this group so they can be cleaned on unregister */
spinlock_t mark_lock; /* protect mark_entries list */
atomic_t num_marks; /* 1 for each mark entry and 1 for not being
@@ -136,11 +143,32 @@ struct fsnotify_group {
};

/*
+ * A single event can be queued in multiple group->notification_lists.
+ *
+ * each group->notification_list will point to an event_holder which in turns points
+ * to the actual event that needs to be sent to userspace.
+ *
+ * Seemed cheaper to create a refcnt'd event and a small holder for every group
+ * than create a different event for every group
+ *
+ */
+struct fsnotify_event_holder {
+ struct fsnotify_event *event;
+ struct list_head event_list;
+};
+
+/*
* all of the information about the original object we want to now send to
* a group. If you want to carry more info from the accessing task to the
* listener this structure is where you need to be adding fields.
*/
struct fsnotify_event {
+ /*
+ * If we create an event we are also likely going to need a holder
+ * to link to a group. So embed one holder in the event. Means only
+ * one allocation for the common case where we only have one group
+ */
+ struct fsnotify_event_holder holder;
spinlock_t lock; /* protection for the associated event_holder and private_list */
/* to_tell may ONLY be dereferenced during handle_event(). */
struct inode *to_tell; /* either the inode the event happened to or its parent */
@@ -264,6 +292,15 @@ extern void fsnotify_put_event(struct fsnotify_event *event);
extern struct fsnotify_event_private_data *fsnotify_get_priv_from_event(struct fsnotify_group *group,
struct fsnotify_event *event);

+/* attach the event to the group notification queue */
+extern int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event);
+/* true if the group notification queue is empty */
+extern bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group);
+/* return, but do not dequeue the first event on the notification queue */
+extern struct fsnotify_event *fsnotify_peek_notify_event(struct fsnotify_group *group);
+/* reutnr AND dequeue the first event on the notification queue */
+extern struct fsnotify_event *fsnotify_remove_notify_event(struct fsnotify_group *group);
+
/* functions used to manipulate the marks attached to inodes */

/* run all marks associated with an inode and update inode->i_fsnotify_mask */


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