fsnotify: Lock object list with connector lock

So far list of marks attached to an object (inode / vfsmount) was
protected by i_lock or mnt_root->d_lock. This dictates that the list
must be empty before the object can be destroyed although the list is
now anchored in the fsnotify_mark_connector structure. Protect the list
by a spinlock in the fsnotify_mark_connector structure to decouple
lifetime of a list of marks from a lifetime of the object. This also
simplifies the code quite a bit since we don't have to differentiate
between inode and vfsmount lists in quite a few places anymore.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 32 insertions, 58 deletions

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index b5b641a2b557..bfb415d0d757 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -33,7 +33,7 @@
  *
  * group->mark_mutex
  * mark->lock
- * inode->i_lock
+ * mark->connector->lock
  *
  * group->mark_mutex protects the marks_list anchored inside a given group and
  * each mark is hooked via the g_list.  It also protects the groups private
@@ -44,10 +44,12 @@
  * is assigned to as well as the access to a reference of the inode/vfsmount
  * that is being watched by the mark.
  *
- * inode->i_lock protects the i_fsnotify_marks list anchored inside a
- * given inode and each mark is hooked via the i_list. (and sorta the
- * free_i_list)
+ * mark->connector->lock protects the list of marks anchored inside an
+ * inode / vfsmount and each mark is hooked via the i_list.
  *
+ * A list of notification marks relating to inode / mnt is contained in
+ * fsnotify_mark_connector. That structure is alive as long as there are any
+ * marks in the list and is also protected by fsnotify_mark_srcu.
  *
  * LIFETIME:
  * Inode marks survive between when they are added to an inode and when their
@@ -110,8 +112,10 @@ static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
         u32 new_mask = 0;
         struct fsnotify_mark *mark;
 
+        assert_spin_locked(&conn->lock);
         hlist_for_each_entry(mark, &conn->list, obj_list)
                 new_mask |= mark->mask;
+
         if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
                 conn->inode->i_fsnotify_mask = new_mask;
         else if (conn->flags & FSNOTIFY_OBJ_TYPE_VFSMOUNT)
@@ -128,31 +132,20 @@ void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
         if (!conn)
                 return;
 
-        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
-                spin_lock(&conn->inode->i_lock);
-        else
-                spin_lock(&conn->mnt->mnt_root->d_lock);
+        spin_lock(&conn->lock);
         __fsnotify_recalc_mask(conn);
-        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE) {
-                spin_unlock(&conn->inode->i_lock);
+        spin_unlock(&conn->lock);
+        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
                 __fsnotify_update_child_dentry_flags(conn->inode);
-        } else {
-                spin_unlock(&conn->mnt->mnt_root->d_lock);
-        }
 }
 
 static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
 {
         struct fsnotify_mark_connector *conn;
         struct inode *inode = NULL;
-        spinlock_t *lock;
 
         conn = mark->connector;
-        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
-                lock = &conn->inode->i_lock;
-        else
-                lock = &conn->mnt->mnt_root->d_lock;
-        spin_lock(lock);
+        spin_lock(&conn->lock);
         hlist_del_init_rcu(&mark->obj_list);
         if (hlist_empty(&conn->list)) {
                 if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
@@ -160,7 +153,7 @@ static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
         }
         __fsnotify_recalc_mask(conn);
         mark->connector = NULL;
-        spin_unlock(lock);
+        spin_unlock(&conn->lock);
 
         return inode;
 }
@@ -326,7 +319,6 @@ int fsnotify_compare_groups(struct fsnotify_group *a, struct fsnotify_group *b)
 
 static int fsnotify_attach_connector_to_object(
                                         struct fsnotify_mark_connector **connp,
-                                        spinlock_t *lock,
                                         struct inode *inode,
                                         struct vfsmount *mnt)
 {
@@ -335,6 +327,7 @@ static int fsnotify_attach_connector_to_object(
         conn = kmem_cache_alloc(fsnotify_mark_connector_cachep, GFP_KERNEL);
         if (!conn)
                 return -ENOMEM;
+        spin_lock_init(&conn->lock);
         INIT_HLIST_HEAD(&conn->list);
         if (inode) {
                 conn->flags = FSNOTIFY_OBJ_TYPE_INODE;
@@ -344,16 +337,13 @@ static int fsnotify_attach_connector_to_object(
                 conn->mnt = mnt;
         }
         /*
-         * Make sure 'conn' initialization is visible. Matches
-         * lockless_dereference() in fsnotify().
+         * cmpxchg() provides the barrier so that readers of *connp can see
+         * only initialized structure
          */
-        smp_wmb();
-        spin_lock(lock);
-        if (!*connp)
-                *connp = conn;
-        else
+        if (cmpxchg(connp, NULL, conn)) {
+                /* Someone else created list structure for us */
                 kmem_cache_free(fsnotify_mark_connector_cachep, conn);
-        spin_unlock(lock);
+        }
 
         return 0;
 }
@@ -371,35 +361,30 @@ static int fsnotify_add_mark_list(struct fsnotify_mark *mark,
         struct fsnotify_mark *lmark, *last = NULL;
         struct fsnotify_mark_connector *conn;
         struct fsnotify_mark_connector **connp;
-        spinlock_t *lock;
         int cmp;
         int err = 0;
 
         if (WARN_ON(!inode && !mnt))
                 return -EINVAL;
-        if (inode) {
+        if (inode)
                 connp = &inode->i_fsnotify_marks;
-                lock = &inode->i_lock;
-        } else {
+        else
                 connp = &real_mount(mnt)->mnt_fsnotify_marks;
-                lock = &mnt->mnt_root->d_lock;
-        }
 
         if (!*connp) {
-                err = fsnotify_attach_connector_to_object(connp, lock,
-                                                          inode, mnt);
+                err = fsnotify_attach_connector_to_object(connp, inode, mnt);
                 if (err)
                         return err;
         }
         spin_lock(&mark->lock);
-        spin_lock(lock);
         conn = *connp;
+        spin_lock(&conn->lock);
 
         /* is mark the first mark? */
         if (hlist_empty(&conn->list)) {
                 hlist_add_head_rcu(&mark->obj_list, &conn->list);
                 if (inode)
-                        __iget(inode);
+                        igrab(inode);
                 goto added;
         }
 
@@ -425,7 +410,7 @@ static int fsnotify_add_mark_list(struct fsnotify_mark *mark,
 added:
         mark->connector = conn;
 out_err:
-        spin_unlock(lock);
+        spin_unlock(&conn->lock);
         spin_unlock(&mark->lock);
         return err;
 }
@@ -449,7 +434,7 @@ int fsnotify_add_mark_locked(struct fsnotify_mark *mark,
          * LOCKING ORDER!!!!
          * group->mark_mutex
          * mark->lock
-         * inode->i_lock
+         * mark->connector->lock
          */
         spin_lock(&mark->lock);
         mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE | FSNOTIFY_MARK_FLAG_ATTACHED;
@@ -505,24 +490,19 @@ struct fsnotify_mark *fsnotify_find_mark(struct fsnotify_mark_connector *conn,
                                          struct fsnotify_group *group)
 {
         struct fsnotify_mark *mark;
-        spinlock_t *lock;
 
         if (!conn)
                 return NULL;
 
-        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
-                lock = &conn->inode->i_lock;
-        else
-                lock = &conn->mnt->mnt_root->d_lock;
-        spin_lock(lock);
+        spin_lock(&conn->lock);
         hlist_for_each_entry(mark, &conn->list, obj_list) {
                 if (mark->group == group) {
                         fsnotify_get_mark(mark);
-                        spin_unlock(lock);
+                        spin_unlock(&conn->lock);
                         return mark;
                 }
         }
-        spin_unlock(lock);
+        spin_unlock(&conn->lock);
         return NULL;
 }
 
@@ -595,16 +575,10 @@ void fsnotify_detach_group_marks(struct fsnotify_group *group)
 void fsnotify_destroy_marks(struct fsnotify_mark_connector *conn)
 {
         struct fsnotify_mark *mark;
-        spinlock_t *lock;
 
         if (!conn)
                 return;
 
-        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
-                lock = &conn->inode->i_lock;
-        else
-                lock = &conn->mnt->mnt_root->d_lock;
-
         while (1) {
                 /*
                  * We have to be careful since we can race with e.g.
@@ -613,15 +587,15 @@ void fsnotify_destroy_marks(struct fsnotify_mark_connector *conn)
                  * we are holding mark reference so mark cannot be freed and
                  * calling fsnotify_destroy_mark() more than once is fine.
                  */
-                spin_lock(lock);
+                spin_lock(&conn->lock);
                 if (hlist_empty(&conn->list)) {
-                        spin_unlock(lock);
+                        spin_unlock(&conn->lock);
                         break;
                 }
                 mark = hlist_entry(conn->list.first, struct fsnotify_mark,
                                    obj_list);
                 fsnotify_get_mark(mark);
-                spin_unlock(lock);
+                spin_unlock(&conn->lock);
                 fsnotify_destroy_mark(mark, mark->group);
                 fsnotify_put_mark(mark);
         }