cifs: delay super block destruction until all cifsFileInfo objects are gone

cifsFileInfo objects hold references to dentries and it is possible that
these will still be around in workqueues when VFS decides to kill super
block during unmount.

This results in panics like this one:
BUG: Dentry ffff88001f5e76c0{i=66b4a,n=1M-2} still in use (1) [unmount of cifs cifs]
------------[ cut here ]------------
kernel BUG at fs/dcache.c:943!
[..]
Process umount (pid: 1781, threadinfo ffff88003d6e8000, task ffff880035eeaec0)
[..]
Call Trace:
 [<ffffffff811b44f3>] shrink_dcache_for_umount+0x33/0x60
 [<ffffffff8119f7fc>] generic_shutdown_super+0x2c/0xe0
 [<ffffffff8119f946>] kill_anon_super+0x16/0x30
 [<ffffffffa036623a>] cifs_kill_sb+0x1a/0x30 [cifs]
 [<ffffffff8119fcc7>] deactivate_locked_super+0x57/0x80
 [<ffffffff811a085e>] deactivate_super+0x4e/0x70
 [<ffffffff811bb417>] mntput_no_expire+0xd7/0x130
 [<ffffffff811bc30c>] sys_umount+0x9c/0x3c0
 [<ffffffff81657c19>] system_call_fastpath+0x16/0x1b

Fix this by making each cifsFileInfo object hold a reference to cifs
super block, which implicitly keeps VFS super block around as well.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Cc: <stable@vger.kernel.org>
Reported-and-Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 8c0d85577314..7a0dd99e4507 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -300,6 +300,8 @@ cifs_new_fileinfo(struct cifs_fid *fid, struct file *file,
         INIT_WORK(&cfile->oplock_break, cifs_oplock_break);
         mutex_init(&cfile->fh_mutex);
 
+        cifs_sb_active(inode->i_sb);
+
         /*
          * If the server returned a read oplock and we have mandatory brlocks,
          * set oplock level to None.
@@ -349,7 +351,8 @@ void cifsFileInfo_put(struct cifsFileInfo *cifs_file)
         struct cifs_tcon *tcon = tlink_tcon(cifs_file->tlink);
         struct TCP_Server_Info *server = tcon->ses->server;
         struct cifsInodeInfo *cifsi = CIFS_I(inode);
-        struct cifs_sb_info *cifs_sb = CIFS_SB(inode->i_sb);
+        struct super_block *sb = inode->i_sb;
+        struct cifs_sb_info *cifs_sb = CIFS_SB(sb);
         struct cifsLockInfo *li, *tmp;
         struct cifs_fid fid;
         struct cifs_pending_open open;
@@ -414,6 +417,7 @@ void cifsFileInfo_put(struct cifsFileInfo *cifs_file)
 
         cifs_put_tlink(cifs_file->tlink);
         dput(cifs_file->dentry);
+        cifs_sb_deactive(sb);
         kfree(cifs_file);
 }