smb: During unmount, ensure all cached dir instances drop their dentry

The unmount process (cifs_kill_sb() calling close_all_cached_dirs()) can
race with various cached directory operations, which ultimately results
in dentries not being dropped and these kernel BUGs:

BUG: Dentry ffff88814f37e358{i=1000000000080,n=/}  still in use (2) [unmount of cifs cifs]
VFS: Busy inodes after unmount of cifs (cifs)
------------[ cut here ]------------
kernel BUG at fs/super.c:661!

This happens when a cfid is in the process of being cleaned up when, and
has been removed from the cfids->entries list, including:

- Receiving a lease break from the server
- Server reconnection triggers invalidate_all_cached_dirs(), which
  removes all the cfids from the list
- The laundromat thread decides to expire an old cfid.

To solve these problems, dropping the dentry is done in queued work done
in a newly-added cfid_put_wq workqueue, and close_all_cached_dirs()
flushes that workqueue after it drops all the dentries of which it's
aware. This is a global workqueue (rather than scoped to a mount), but
the queued work is minimal.

The final cleanup work for cleaning up a cfid is performed via work
queued in the serverclose_wq workqueue; this is done separate from
dropping the dentries so that close_all_cached_dirs() doesn't block on
any server operations.

Both of these queued works expect to invoked with a cfid reference and
a tcon reference to avoid those objects from being freed while the work
is ongoing.

While we're here, add proper locking to close_all_cached_dirs(), and
locking around the freeing of cfid->dentry.

Fixes: ebe98f1447bb ("cifs: enable caching of directories for which a lease is held")
Cc: stable@vger.kernel.org
Signed-off-by: Paul Aurich <paul@darkrain42.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

1 files changed, 11 insertions, 1 deletions

diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index 979853471027..c9f9b6e97964 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -157,6 +157,7 @@ struct workqueue_struct	*fileinfo_put_wq;
 struct workqueue_struct	*cifsoplockd_wq;
 struct workqueue_struct	*deferredclose_wq;
 struct workqueue_struct	*serverclose_wq;
+struct workqueue_struct	*cfid_put_wq;
 __u32 cifs_lock_secret;
 
 /*
@@ -1920,9 +1921,16 @@ init_cifs(void)
 		goto out_destroy_deferredclose_wq;
 	}
 
+	cfid_put_wq = alloc_workqueue("cfid_put_wq",
+				      WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
+	if (!cfid_put_wq) {
+		rc = -ENOMEM;
+		goto out_destroy_serverclose_wq;
+	}
+
 	rc = cifs_init_inodecache();
 	if (rc)
-		goto out_destroy_serverclose_wq;
+		goto out_destroy_cfid_put_wq;
 
 	rc = cifs_init_netfs();
 	if (rc)
@@ -1990,6 +1998,8 @@ out_destroy_netfs:
 	cifs_destroy_netfs();
 out_destroy_inodecache:
 	cifs_destroy_inodecache();
+out_destroy_cfid_put_wq:
+	destroy_workqueue(cfid_put_wq);
 out_destroy_serverclose_wq:
 	destroy_workqueue(serverclose_wq);
 out_destroy_deferredclose_wq: