[PATCH -V2 03/13] fsnotify: add group priorities

[Eric Paris]

This introduces an ordering to fnotify groups.  It's most interesting because
an HSM would need to run before a typical access notifier.  And an access
control system would need to run between the two.

Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
---

 fs/notify/group.c                |   29 ++++++++++++++++++++++-------
 include/linux/fsnotify_backend.h |    4 +++-
 2 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/fs/notify/group.c b/fs/notify/group.c
index 88d040b..dd1d18d 100644
--- a/fs/notify/group.c
+++ b/fs/notify/group.c
@@ -49,8 +49,21 @@ void fsnotify_recalc_global_mask(void)
 
 static void fsnotify_add_group(struct fsnotify_group *group)
 {
-	list_add_rcu(&group->group_list, &fsnotify_groups);
+	int priority = group->priority;
+	struct fsnotify_group *group_iter;
+
 	group->evicted = 0;
+	list_for_each_entry(group_iter, &fsnotify_groups, group_list) {
+		/* insert in front of this one? */
+		if (priority < group_iter->priority) {
+			/* I used list_add_tail() to insert in front of group_iter...  */
+			list_add_tail_rcu(&group->group_list, &group_iter->group_list);
+			return;
+		}
+	}
+
+	/* apparently we need to be the last entry */
+	list_add_tail_rcu(&group->group_list, &fsnotify_groups);
 }
 
 static void fsnotify_get_group(struct fsnotify_group *group)
@@ -101,8 +114,8 @@ void fsnotify_put_group(struct fsnotify_group *group)
 	fsnotify_destroy_group(group);
 }
 
-static struct fsnotify_group *fsnotify_find_group(unsigned int group_num, __u32 mask,
-						  const struct fsnotify_ops *ops)
+static struct fsnotify_group *fsnotify_find_group(unsigned int priority, unsigned int group_num,
+						  __u32 mask, const struct fsnotify_ops *ops)
 {
 	struct fsnotify_group *group_iter;
 	struct fsnotify_group *group = NULL;
@@ -110,8 +123,9 @@ static struct fsnotify_group *fsnotify_find_group(unsigned int group_num, __u32
 	BUG_ON(!mutex_is_locked(&fsnotify_grp_mutex));
 
 	list_for_each_entry_rcu(group_iter, &fsnotify_groups, group_list) {
-		if (group_iter->group_num == group_num) {
+		if (group_iter->priority == priority) {
 			if ((group_iter->mask == mask) &&
+			    (group_iter->group_num == group_num) &&
 			    (group_iter->ops == ops)) {
 				fsnotify_get_group(group_iter);
 				group = group_iter;
@@ -131,8 +145,8 @@ static struct fsnotify_group *fsnotify_find_group(unsigned int group_num, __u32
  * the allocation and the initialization, but this is only called when notification
  * systems make changes, so why make it more complex?
  */
-struct fsnotify_group *fsnotify_obtain_group(unsigned int group_num, __u32 mask,
-					     const struct fsnotify_ops *ops)
+struct fsnotify_group *fsnotify_obtain_group(unsigned int priority, unsigned int group_num,
+					     __u32 mask, const struct fsnotify_ops *ops)
 {
 	struct fsnotify_group *group, *tgroup;
 
@@ -142,13 +156,14 @@ struct fsnotify_group *fsnotify_obtain_group(unsigned int group_num, __u32 mask,
 
 	atomic_set(&group->refcnt, 1);
 
+	group->priority = priority;
 	group->group_num = group_num;
 	group->mask = mask;
 
 	group->ops = ops;
 
 	mutex_lock(&fsnotify_grp_mutex);
-	tgroup = fsnotify_find_group(group_num, mask, ops);
+	tgroup = fsnotify_find_group(priority, group_num, mask, ops);
 	if (tgroup) {
 		/* group already exists */
 		mutex_unlock(&fsnotify_grp_mutex);
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 0523333..a349691 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -76,6 +76,7 @@ struct fsnotify_group {
 
 	const struct fsnotify_ops *ops;	/* how this group handles things */
 
+	unsigned int priority;		/* order this group should receive msgs.  low first */
 	unsigned int evicted:1;		/* has this group been evicted? */
 
 	/* groups can define private fields here */
@@ -115,7 +116,8 @@ extern void fsnotify(struct inode *to_tell, __u32 mask, void *data, int data_is)
 
 /* called from fsnotify interfaces, such as fanotify or dnotify */
 extern void fsnotify_recalc_global_mask(void);
-extern struct fsnotify_group *fsnotify_obtain_group(unsigned int group_num, __u32 mask, const struct fsnotify_ops *ops);
+extern struct fsnotify_group *fsnotify_obtain_group(unsigned int priority, unsigned int group_num,
+						    __u32 mask, const struct fsnotify_ops *ops);
 extern void fsnotify_put_group(struct fsnotify_group *group);
 
 extern void fsnotify_get_event(struct fsnotify_event *event);

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