[PATCH] cifs: Fix busy dentry used after unmounting

Since commit 340cea84f691c ("cifs: open files should not hold ref on
superblock"), cifs file only holds the dentry ref_cnt, the cifs file
close work(cfile->deferred) could be executed after unmounting, which
will trigger a warning in generic_shutdown_super:
 BUG: Dentry 00000000a14a6845{i=c,n=file}  still in use (1) [unmount of
 cifs cifs]

The detailed processs is:
   process A           process B           kworker
 fd = open(PATH)
  vfs_open
   file->__f_path = *path // dentry->d_lockref.count = 1
   cifs_open
    cifs_new_fileinfo
     cfile->dentry = dget(dentry) // dentry->d_lockref.count = 2
 close(fd)
  __fput
  cifs_close
   queue_delayed_work(deferredclose_wq, cfile->deferred)
  dput(dentry) // dentry->d_lockref.count = 1
			                 smb2_deferred_work_close
					  _cifsFileInfo_put
					   list_del(&cifs_file->flist)
                    umount
		     cleanup_mnt
		      deactivate_super
		       cifs_kill_sb
		        cifs_close_all_deferred_files_sb
			 cifs_close_all_deferred_files
			  // cannot find cfile, skip _cifsFileInfo_put
			kill_anon_super
			 generic_shutdown_super
			  shrink_dcache_for_umount
			   umount_check
			    WARN ! // dentry->d_lockref.count = 1
					   cifsFileInfo_put_final
					    dput(cifs_file->dentry)
		                            // dentry->d_lockref.count = 0

Fix it by flushing 'deferredclose_wq' before calling kill_anon_super.

Fetch a reproducer in https://bugzilla.kernel.org/show_bug.cgi?id=221548.

Fixes: 340cea84f691c ("cifs: open files should not hold ref on superblock")
Signed-off-by: Zhihao Cheng <[email protected]>
---
 fs/smb/client/cifsfs.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index f557eb7875c7..ce23924f01b3 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -306,6 +306,8 @@ static void cifs_kill_sb(struct super_block *sb)
 
 		/* Wait for all pending oplock breaks to complete */
 		flush_workqueue(cifsoplockd_wq);
+		/* Wait for all opened files to release */
+		flush_workqueue(deferredclose_wq);
 
 		/* finally release root dentry */
 		dput(cifs_sb->root);
-- 
2.52.0