vfs: check submounts and drop atomically
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@suse.cz>
CC: David Howells <dhowells@redhat.com>
CC: Steven Whitehouse <swhiteho@redhat.com>
CC: Trond Myklebust <Trond.Myklebust@netapp.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/dcache.c b/fs/dcache.c
index 043c5b4..ce5a7e6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1264,6 +1264,71 @@
}
EXPORT_SYMBOL(shrink_dcache_parent);
+static enum d_walk_ret check_and_collect(void *_data, struct dentry *dentry)
+{
+ struct select_data *data = _data;
+
+ if (d_mountpoint(dentry)) {
+ data->found = -EBUSY;
+ return D_WALK_QUIT;
+ }
+
+ return select_collect(_data, dentry);
+}
+
+static void check_and_drop(void *_data)
+{
+ struct select_data *data = _data;
+
+ if (d_mountpoint(data->start))
+ data->found = -EBUSY;
+ if (!data->found)
+ __d_drop(data->start);
+}
+
+/**
+ * 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;
+ }
+
+ for (;;) {
+ struct select_data data;
+
+ INIT_LIST_HEAD(&data.dispose);
+ data.start = dentry;
+ data.found = 0;
+
+ d_walk(dentry, &data, check_and_collect, check_and_drop);
+ ret = data.found;
+
+ if (!list_empty(&data.dispose))
+ shrink_dentry_list(&data.dispose);
+
+ if (ret <= 0)
+ break;
+
+ cond_resched();
+ }
+
+out:
+ return ret;
+}
+EXPORT_SYMBOL(check_submounts_and_drop);
+
/**
* __d_alloc - allocate a dcache entry
* @sb: filesystem it will belong to