[PATCH 1/9] vfs: check submounts and drop atomically

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@xxxxxxx>

We check submounts before doing d_drop() on a non-empty directory dentry in
NFS (have_submounts()), but we do not exclude a racing mount.

 Process A: have_submounts() -> returns false
 Process B: mount() -> success
 Process A: d_drop()

This patch prepares the ground for the fix by doing the following
operations all under the same rename lock:

  have_submounts()
  shrink_dcache_parent()
  d_drop()

This is actually an optimization since have_submounts() and
shrink_dcache_parent() both traverse the same dentry tree separately.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
CC: David Howells <dhowells@xxxxxxxxxx>
CC: Steven Whitehouse <swhiteho@xxxxxxxxxx>
CC: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/dcache.c            | 157 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/dcache.h |   1 +
 2 files changed, 158 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 87bdb53..020004d 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1224,6 +1224,163 @@ void shrink_dcache_parent(struct dentry * parent)
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
 
+static int __check_submounts_and_drop(struct dentry *parent,
+					     struct list_head *dispose)
+{
+	struct dentry *this_parent;
+	struct list_head *next;
+	unsigned seq;
+	int found = 0;
+	int locked = 0;
+
+	seq = read_seqbegin(&rename_lock);
+again:
+	this_parent = parent;
+	spin_lock(&this_parent->d_lock);
+repeat:
+	next = this_parent->d_subdirs.next;
+resume:
+	while (next != &this_parent->d_subdirs) {
+		struct list_head *tmp = next;
+		struct dentry *dentry;
+
+		dentry = list_entry(tmp, struct dentry, d_u.d_child);
+		next = tmp->next;
+
+		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+		if (d_mountpoint(dentry)) {
+			spin_unlock(&dentry->d_lock);
+			found = -EBUSY;
+			goto out;
+		}
+
+		/*
+		 * move only zero ref count dentries to the dispose list.
+		 *
+		 * Those which are presently on the shrink list, being processed
+		 * by shrink_dentry_list(), shouldn't be moved.  Otherwise the
+		 * loop in shrink_dcache_parent() might not make any progress
+		 * and loop forever.
+		 */
+		if (dentry->d_count) {
+			dentry_lru_del(dentry);
+		} else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
+			dentry_lru_move_list(dentry, dispose);
+			dentry->d_flags |= DCACHE_SHRINK_LIST;
+			found++;
+		}
+		/*
+		 * We can return to the caller if we have found some (this
+		 * ensures forward progress). We'll be coming back to find
+		 * the rest.
+		 */
+		if (found && need_resched()) {
+			spin_unlock(&dentry->d_lock);
+			goto out;
+		}
+
+		/*
+		 * Descend a level if the d_subdirs list is non-empty.
+		 */
+		if (!list_empty(&dentry->d_subdirs)) {
+			spin_unlock(&this_parent->d_lock);
+			spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);
+			this_parent = dentry;
+			spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
+			goto repeat;
+		}
+
+		spin_unlock(&dentry->d_lock);
+	}
+	/*
+	 * All done at this level ... ascend and resume the search.
+	 */
+	if (this_parent != parent) {
+		struct dentry *child = this_parent;
+		this_parent = try_to_ascend(this_parent, locked, seq);
+		if (!this_parent)
+			goto rename_retry;
+		next = child->d_u.d_child.next;
+		goto resume;
+	}
+	if (!locked && read_seqretry(&rename_lock, seq)) {
+		spin_unlock(&this_parent->d_lock);
+		goto rename_retry;
+	}
+	if (d_mountpoint(this_parent))
+		found = -EBUSY;
+	if (!found)
+		__d_drop(this_parent);
+out:
+	spin_unlock(&this_parent->d_lock);
+
+	if (locked)
+		write_sequnlock(&rename_lock);
+	return found;
+
+rename_retry:
+	if (found)
+		return found;
+	if (locked)
+		goto again;
+	locked = 1;
+	write_seqlock(&rename_lock);
+	goto again;
+}
+
+/**
+ * check_submounts_and_drop - prune dcache, check for submounts and drop
+ *
+ * All done as a single atomic operation relative to has_unlinked_ancestor().
+ * Returns 0 if successfully unhashed @parent.  If there were submounts then
+ * return -EBUSY.
+ *
+ * @dentry: dentry to prune and drop
+ */
+int check_submounts_and_drop(struct dentry *dentry)
+{
+	int ret = 0;
+
+	/* Negative dentries can be dropped without further checks */
+	if (!dentry->d_inode) {
+		d_drop(dentry);
+		goto out;
+	}
+
+	spin_lock(&dentry->d_lock);
+	if (d_unhashed(dentry))
+		goto out_unlock;
+	if (list_empty(&dentry->d_subdirs)) {
+		if (d_mountpoint(dentry)) {
+			ret = -EBUSY;
+			goto out_unlock;
+		}
+		__d_drop(dentry);
+		goto out_unlock;
+	}
+	spin_unlock(&dentry->d_lock);
+
+	for (;;) {
+		LIST_HEAD(dispose);
+		ret = __check_submounts_and_drop(dentry, &dispose);
+		if (!list_empty(&dispose))
+			shrink_dentry_list(&dispose);
+
+		if (ret <= 0)
+			break;
+
+		cond_resched();
+	}
+
+out:
+	return ret;
+
+out_unlock:
+	spin_unlock(&dentry->d_lock);
+	goto out;
+}
+EXPORT_SYMBOL(check_submounts_and_drop);
+
 /**
  * __d_alloc	-	allocate a dcache entry
  * @sb: filesystem it will belong to
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index b90337c..41b21ca 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -251,6 +251,7 @@ extern void d_prune_aliases(struct inode *);
 
 /* test whether we have any submounts in a subdir tree */
 extern int have_submounts(struct dentry *);
+extern int check_submounts_and_drop(struct dentry *);
 
 /*
  * This adds the entry to the hash queues.
-- 
1.8.1.4

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