[PATCH] smb: client: fix busy dentry warning on unmount after DIO

[Zizhi Wo]

From: Zizhi Wo <wozizhi@xxxxxxxxxx>

Commit c68337442f03 ("cifs: Fix busy dentry used after unmounting") fixed
the issue in cifs where deferred close of a file led to a dentry reference
count not being released in umount, by flushing deferredclose_wq in
cifs_kill_sb() to solve it.

However, the cifs DIO path suffers from the same busy-dentry problem caused
by a delayed dentry reference-count release:

	[dio]			[cifsd]			[close + umount]
netfs_unbuffered_write_iter_locked
...
				cifs_demultiplex_thread
 netfs_unbuffered_write
  cifs_issue_write
  netfs_wait_for_in_progress_stream [1]
				...
				 netfs_write_subrequest_terminated
				  netfs_subreq_clear_in_progress
				   netfs_wake_collector // wake [1]
				  netfs_put_subrequest
 netfs_put_request
  queue_work(system_dfl_wq, xxx) [2]
 // dio write return					cifs_close
							 _cifsFileInfo_put
							  // cfile->count 2->1
							  --cfile->count [3]

							// umount
							cifs_kill_sb
							 kill_anon_super
							  // warning triggered!
							  shrink_dcache_for_umount [4]
[system_dfl_wq] [5]
netfs_free_request
 ...
 _cifsFileInfo_put
  // cfile->count 1->0
  --cfile->count
  queue_work(fileinfo_put_wq, xxx)

[fileinfo_put_wq] [6]
cifsFileInfo_put_work
 cifsFileInfo_put_final
  dput

If the umount path is triggered before [5], it results warning:
BUG: Dentry 00000000eab1f070{i=9a917b66ae404fec,n=test}  still in use (1)
[unmount of cifs cifs]

The existing per-inode ictx->io_count wait in cifs_evict_inode() does not
help: it lives in the inode eviction path, which runs after
shrink_dcache_for_umount() has already warned about the busy dentries.

Fix it by adding a per-superblock outstanding-rreq counter that is
incremented in cifs_init_request() and decremented in cifs_free_request().
In cifs_kill_sb(), before kill_anon_super(), wait for this counter to reach
0 - which guarantees that all cleanup_work for this sb have run and thus
all relevant cfile puts are queued on fileinfo_put_wq. Then drain
fileinfo_put_wq so the dentry refs are dropped.

This is a targeted wait, not a flush of the system-wide system_dfl_wq.

Fixes: 340cea84f691c ("cifs: open files should not hold ref on superblock")
Signed-off-by: Zizhi Wo <wozizhi@xxxxxxxxxx>
---
 fs/smb/client/cifs_fs_sb.h |  1 +
 fs/smb/client/cifsfs.c     | 11 +++++++++++
 fs/smb/client/connect.c    |  1 +
 fs/smb/client/file.c       |  5 +++++
 4 files changed, 18 insertions(+)

diff --git a/fs/smb/client/cifs_fs_sb.h b/fs/smb/client/cifs_fs_sb.h
index 84e7e366b0ff..d6494e1d93cc 100644
--- a/fs/smb/client/cifs_fs_sb.h
+++ b/fs/smb/client/cifs_fs_sb.h
@@ -54,10 +54,11 @@ struct cifs_sb_info {
 	struct tcon_link *master_tlink;
 	struct nls_table *local_nls;
 	struct smb3_fs_context *ctx;
 	atomic_t active;
 	atomic_t mnt_cifs_flags;
+	atomic_t outstanding_rreq;	/* nr of rreqs not yet fully deinitialized */
 	struct delayed_work prune_tlinks;
 	struct rcu_head rcu;
 
 	/* only used when CIFS_MOUNT_USE_PREFIX_PATH is set */
 	char *prepath;
diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index ea4fc0fa68ca..6696893d522f 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -309,10 +309,21 @@ 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);
 
+		/*
+		 * Wait for all in-flight netfs I/O requests to finish their
+		 * cleanup_work so that any cifsFileInfo final puts they queue
+		 * to fileinfo_put_wq have been queued, then drain that
+		 * workqueue so the cfile dentry refs are dropped to avoid the
+		 * busy dentry warning.
+		 */
+		wait_var_event(&cifs_sb->outstanding_rreq,
+			       !atomic_read(&cifs_sb->outstanding_rreq));
+		flush_workqueue(fileinfo_put_wq);
+
 		/* finally release root dentry */
 		dput(cifs_sb->root);
 		cifs_sb->root = NULL;
 	}
 
diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index 85aec302c89e..a187398fbabd 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -3483,10 +3483,11 @@ int cifs_setup_cifs_sb(struct cifs_sb_info *cifs_sb)
 	INIT_DELAYED_WORK(&cifs_sb->prune_tlinks, cifs_prune_tlinks);
 	INIT_LIST_HEAD(&cifs_sb->tcon_sb_link);
 
 	spin_lock_init(&cifs_sb->tlink_tree_lock);
 	cifs_sb->tlink_tree = RB_ROOT;
+	atomic_set(&cifs_sb->outstanding_rreq, 0);
 
 	cifs_dbg(FYI, "file mode: %04ho  dir mode: %04ho\n",
 		 ctx->file_mode, ctx->dir_mode);
 
 	/* this is needed for ASCII cp to Unicode converts */
diff --git a/fs/smb/client/file.c b/fs/smb/client/file.c
index 8b25d6c9ec5e..d2116d934466 100644
--- a/fs/smb/client/file.c
+++ b/fs/smb/client/file.c
@@ -285,10 +285,11 @@ static int cifs_init_request(struct netfs_io_request *rreq, struct file *file)
 	} else if (rreq->origin != NETFS_WRITEBACK) {
 		WARN_ON_ONCE(1);
 		return smb_EIO1(smb_eio_trace_not_netfs_writeback, rreq->origin);
 	}
 
+	atomic_inc(&cifs_sb->outstanding_rreq);
 	return 0;
 }
 
 /*
  * Completion of a request operation.
@@ -306,13 +307,17 @@ static void cifs_rreq_done(struct netfs_io_request *rreq)
 }
 
 static void cifs_free_request(struct netfs_io_request *rreq)
 {
 	struct cifs_io_request *req = container_of(rreq, struct cifs_io_request, rreq);
+	struct cifs_sb_info *cifs_sb = CIFS_SB(rreq->inode->i_sb);
 
 	if (req->cfile)
 		cifsFileInfo_put(req->cfile);
+
+	if (atomic_dec_and_test(&cifs_sb->outstanding_rreq))
+		wake_up_var(&cifs_sb->outstanding_rreq);
 }
 
 static void cifs_free_subrequest(struct netfs_io_subrequest *subreq)
 {
 	struct cifs_io_subrequest *rdata =
-- 
2.52.0