fsnotify: avoid spurious EMFILE errors from inotify_init()

Inotify instance is destroyed when all references to it are dropped.
That not only means that the corresponding file descriptor needs to be
closed but also that all corresponding instance marks are freed (as each
mark holds a reference to the inotify instance).  However marks are
freed only after SRCU period ends which can take some time and thus if
user rapidly creates and frees inotify instances, number of existing
inotify instances can exceed max_user_instances limit although from user
point of view there is always at most one existing instance.  Thus
inotify_init() returns EMFILE error which is hard to justify from user
point of view.  This problem is exposed by LTP inotify06 testcase on
some machines.

We fix the problem by making sure all group marks are properly freed
while destroying inotify instance.  We wait for SRCU period to end in
that path anyway since we have to make sure there is no event being
added to the instance while we are tearing down the instance.  So it
takes only some plumbing to allow for marks to be destroyed in that path
as well and not from a dedicated work item.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Tested-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 4 deletions

diff --git a/fs/notify/group.c b/fs/notify/group.c
index d16b62cb2854..3e2dd85be5dd 100644
--- a/fs/notify/group.c
+++ b/fs/notify/group.c
@@ -47,12 +47,21 @@ static void fsnotify_final_destroy_group(struct fsnotify_group *group)
  */
 void fsnotify_destroy_group(struct fsnotify_group *group)
 {
-	/* clear all inode marks for this group */
-	fsnotify_clear_marks_by_group(group);
+	/* clear all inode marks for this group, attach them to destroy_list */
+	fsnotify_detach_group_marks(group);
 
-	synchronize_srcu(&fsnotify_mark_srcu);
+	/*
+	 * Wait for fsnotify_mark_srcu period to end and free all marks in
+	 * destroy_list
+	 */
+	fsnotify_mark_destroy_list();
 
-	/* clear the notification queue of all events */
+	/*
+	 * Since we have waited for fsnotify_mark_srcu in
+	 * fsnotify_mark_destroy_list() there can be no outstanding event
+	 * notification against this group. So clearing the notification queue
+	 * of all events is reliable now.
+	 */
 	fsnotify_flush_notify(group);
 
 	/*