[patch 001/117] fsnotify: avoid spurious EMFILE errors from inotify_init()

[patch 001/117] fsnotify: avoid spurious EMFILE errors from inotify_init()

[akpm]

From: Jan Kara <jack@suse.cz>
Subject: 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>
---

 fs/notify/fsnotify.h             |    7 ++
 fs/notify/group.c                |   17 ++++--
 fs/notify/mark.c                 |   78 ++++++++++++++++++++++-------
 include/linux/fsnotify_backend.h |    2 
 4 files changed, 81 insertions(+), 23 deletions(-)

diff -puN fs/notify/fsnotify.h~fsnotify-avoid-spurious-emfile-errors-from-inotify_init fs/notify/fsnotify.h
--- a/fs/notify/fsnotify.h~fsnotify-avoid-spurious-emfile-errors-from-inotify_init
+++ a/fs/notify/fsnotify.h
@@ -56,6 +56,13 @@ static inline void fsnotify_clear_marks_
 	fsnotify_destroy_marks(&real_mount(mnt)->mnt_fsnotify_marks,
 			       &mnt->mnt_root->d_lock);
 }
+/* prepare for freeing all marks associated with given group */
+extern void fsnotify_detach_group_marks(struct fsnotify_group *group);
+/*
+ * wait for fsnotify_mark_srcu period to end and free all marks in destroy_list
+ */
+extern void fsnotify_mark_destroy_list(void);
+
 /*
  * update the dentry->d_flags of all of inode's children to indicate if inode cares
  * about events that happen to its children.
diff -puN fs/notify/group.c~fsnotify-avoid-spurious-emfile-errors-from-inotify_init fs/notify/group.c
--- a/fs/notify/group.c~fsnotify-avoid-spurious-emfile-errors-from-inotify_init
+++ a/fs/notify/group.c
@@ -47,12 +47,21 @@ static void fsnotify_final_destroy_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);
 
 	/*
diff -puN fs/notify/mark.c~fsnotify-avoid-spurious-emfile-errors-from-inotify_init fs/notify/mark.c
--- a/fs/notify/mark.c~fsnotify-avoid-spurious-emfile-errors-from-inotify_init
+++ a/fs/notify/mark.c
@@ -97,8 +97,8 @@ struct srcu_struct fsnotify_mark_srcu;
 static DEFINE_SPINLOCK(destroy_lock);
 static LIST_HEAD(destroy_list);
 
-static void fsnotify_mark_destroy(struct work_struct *work);
-static DECLARE_DELAYED_WORK(reaper_work, fsnotify_mark_destroy);
+static void fsnotify_mark_destroy_workfn(struct work_struct *work);
+static DECLARE_DELAYED_WORK(reaper_work, fsnotify_mark_destroy_workfn);
 
 void fsnotify_get_mark(struct fsnotify_mark *mark)
 {
@@ -173,11 +173,15 @@ void fsnotify_detach_mark(struct fsnotif
 }
 
 /*
- * Free fsnotify mark. The freeing is actually happening from a kthread which
- * first waits for srcu period end. Caller must have a reference to the mark
- * or be protected by fsnotify_mark_srcu.
+ * Prepare mark for freeing and add it to the list of marks prepared for
+ * freeing. The actual freeing must happen after SRCU period ends and the
+ * caller is responsible for this.
+ *
+ * The function returns true if the mark was added to the list of marks for
+ * freeing. The function returns false if someone else has already called
+ * __fsnotify_free_mark() for the mark.
  */
-void fsnotify_free_mark(struct fsnotify_mark *mark)
+static bool __fsnotify_free_mark(struct fsnotify_mark *mark)
 {
 	struct fsnotify_group *group = mark->group;
 
@@ -185,17 +189,11 @@ void fsnotify_free_mark(struct fsnotify_
 	/* something else already called this function on this mark */
 	if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
 		spin_unlock(&mark->lock);
-		return;
+		return false;
 	}
 	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 	spin_unlock(&mark->lock);
 
-	spin_lock(&destroy_lock);
-	list_add(&mark->g_list, &destroy_list);
-	spin_unlock(&destroy_lock);
-	queue_delayed_work(system_unbound_wq, &reaper_work,
-				FSNOTIFY_REAPER_DELAY);
-
 	/*
 	 * Some groups like to know that marks are being freed.  This is a
 	 * callback to the group function to let it know that this mark
@@ -203,6 +201,25 @@ void fsnotify_free_mark(struct fsnotify_
 	 */
 	if (group->ops->freeing_mark)
 		group->ops->freeing_mark(mark, group);
+
+	spin_lock(&destroy_lock);
+	list_add(&mark->g_list, &destroy_list);
+	spin_unlock(&destroy_lock);
+
+	return true;
+}
+
+/*
+ * Free fsnotify mark. The freeing is actually happening from a workqueue which
+ * first waits for srcu period end. Caller must have a reference to the mark
+ * or be protected by fsnotify_mark_srcu.
+ */
+void fsnotify_free_mark(struct fsnotify_mark *mark)
+{
+	if (__fsnotify_free_mark(mark)) {
+		queue_delayed_work(system_unbound_wq, &reaper_work,
+				   FSNOTIFY_REAPER_DELAY);
+	}
 }
 
 void fsnotify_destroy_mark(struct fsnotify_mark *mark,
@@ -468,11 +485,29 @@ void fsnotify_clear_marks_by_group_flags
 }
 
 /*
- * Given a group, destroy all of the marks associated with that group.
+ * Given a group, prepare for freeing all the marks associated with that group.
+ * The marks are attached to the list of marks prepared for destruction, the
+ * caller is responsible for freeing marks in that list after SRCU period has
+ * ended.
  */
-void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
+void fsnotify_detach_group_marks(struct fsnotify_group *group)
 {
-	fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1);
+	struct fsnotify_mark *mark;
+
+	while (1) {
+		mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
+		if (list_empty(&group->marks_list)) {
+			mutex_unlock(&group->mark_mutex);
+			break;
+		}
+		mark = list_first_entry(&group->marks_list,
+					struct fsnotify_mark, g_list);
+		fsnotify_get_mark(mark);
+		fsnotify_detach_mark(mark);
+		mutex_unlock(&group->mark_mutex);
+		__fsnotify_free_mark(mark);
+		fsnotify_put_mark(mark);
+	}
 }
 
 void fsnotify_duplicate_mark(struct fsnotify_mark *new, struct fsnotify_mark *old)
@@ -499,7 +534,11 @@ void fsnotify_init_mark(struct fsnotify_
 	mark->free_mark = free_mark;
 }
 
-static void fsnotify_mark_destroy(struct work_struct *work)
+/*
+ * Destroy all marks in destroy_list, waits for SRCU period to finish before
+ * actually freeing marks.
+ */
+void fsnotify_mark_destroy_list(void)
 {
 	struct fsnotify_mark *mark, *next;
 	struct list_head private_destroy_list;
@@ -516,3 +555,8 @@ static void fsnotify_mark_destroy(struct
 		fsnotify_put_mark(mark);
 	}
 }
+
+static void fsnotify_mark_destroy_workfn(struct work_struct *work)
+{
+	fsnotify_mark_destroy_list();
+}
diff -puN include/linux/fsnotify_backend.h~fsnotify-avoid-spurious-emfile-errors-from-inotify_init include/linux/fsnotify_backend.h
--- a/include/linux/fsnotify_backend.h~fsnotify-avoid-spurious-emfile-errors-from-inotify_init
+++ a/include/linux/fsnotify_backend.h
@@ -359,8 +359,6 @@ extern void fsnotify_clear_vfsmount_mark
 extern void fsnotify_clear_inode_marks_by_group(struct fsnotify_group *group);
 /* run all the marks in a group, and clear all of the marks where mark->flags & flags is true*/
 extern void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group, unsigned int flags);
-/* run all the marks in a group, and flag them to be freed */
-extern void fsnotify_clear_marks_by_group(struct fsnotify_group *group);
 extern void fsnotify_get_mark(struct fsnotify_mark *mark);
 extern void fsnotify_put_mark(struct fsnotify_mark *mark);
 extern void fsnotify_unmount_inodes(struct super_block *sb);
_