Re: [PATCH] smb: client: avoid leaking refcount in cifs_queue_oplock_break()

[Paulo Alcantara <pc@manguebit.org>]

Bjoern Doebel <doebel@amazon.de> writes:

> cifs_queue_oplock_break() unconditionally takes a reference on the
> target file before queueing cifs_oplock_break(). Only that work item
> decreases the reference counter again.
>
> If another oplock break arrives while that work is still queued,
> queue_work() will return false and not queue this second work item. As a
> result, we will never reach the point to drop the file reference again
> and are leaking this reference. This can be triggered when interacting
> with a slow-responding server.
>
> As a result, later unmount operations for this file system will fail with
>
>   BUG: Dentry ... still in use (1) [unmount of cifs cifs]
>   VFS: Busy inodes after unmount of cifs (cifs)
>   kernel BUG at fs/super.c:777!
>
> Fix this by only incrementing the reference count if the work has been
> queued successfully. Taking it after queue_work() is safe because all
> three callers hold tcon->open_file_lock across the call and
> _cifsFileInfo_put() decrements under that same lock, so a worker that
> starts the handler in the window cannot drop the reference before it has
> been taken.
> ...

Applied.