NFSv4: Ensure the delegation is pinned in nfs_do_return_delegation()

The call to nfs_do_return_delegation() needs to be taken without
any RCU locks. Add a refcount to make sure the delegation remains
pinned in memory until we're done.

Fixes: ee05f456772d ("NFSv4: Fix races between open and delegreturn")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
index d856326..c17ff82 100644
--- a/fs/nfs/delegation.c
+++ b/fs/nfs/delegation.c
@@ -47,10 +47,22 @@
 	}
 }
 
+static struct nfs_delegation *nfs_get_delegation(struct nfs_delegation *delegation)
+{
+	refcount_inc(&delegation->refcount);
+	return delegation;
+}
+
+static void nfs_put_delegation(struct nfs_delegation *delegation)
+{
+	if (refcount_dec_and_test(&delegation->refcount))
+		__nfs_free_delegation(delegation);
+}
+
 static void nfs_free_delegation(struct nfs_delegation *delegation)
 {
 	nfs_mark_delegation_revoked(delegation);
-	__nfs_free_delegation(delegation);
+	nfs_put_delegation(delegation);
 }
 
 /**
@@ -275,8 +287,10 @@
 	if (delegation == NULL)
 		goto out;
 	spin_lock(&delegation->lock);
-	if (!test_and_set_bit(NFS_DELEGATION_RETURNING, &delegation->flags))
-		ret = delegation;
+	if (!test_and_set_bit(NFS_DELEGATION_RETURNING, &delegation->flags)) {
+		/* Refcount matched in nfs_end_delegation_return() */
+		ret = nfs_get_delegation(delegation);
+	}
 	spin_unlock(&delegation->lock);
 	if (ret)
 		nfs_clear_verifier_delegated(&nfsi->vfs_inode);
@@ -397,6 +411,7 @@
 	if (delegation == NULL)
 		return -ENOMEM;
 	nfs4_stateid_copy(&delegation->stateid, stateid);
+	refcount_set(&delegation->refcount, 1);
 	delegation->type = type;
 	delegation->pagemod_limit = pagemod_limit;
 	delegation->change_attr = inode_peek_iversion_raw(inode);
@@ -496,6 +511,8 @@
 
 	err = nfs_do_return_delegation(inode, delegation, issync);
 out:
+	/* Refcount matched in nfs_start_delegation_return_locked() */
+	nfs_put_delegation(delegation);
 	return err;
 }
 
@@ -690,7 +707,8 @@
 		    list_empty(&NFS_I(inode)->open_files) &&
 		    !test_and_set_bit(NFS_DELEGATION_RETURNING, &delegation->flags)) {
 			clear_bit(NFS_DELEGATION_RETURN_IF_CLOSED, &delegation->flags);
-			ret = delegation;
+			/* Refcount matched in nfs_end_delegation_return() */
+			ret = nfs_get_delegation(delegation);
 		}
 		spin_unlock(&delegation->lock);
 		if (ret)
@@ -1094,10 +1112,11 @@
 			delegation = nfs_start_delegation_return_locked(NFS_I(inode));
 			rcu_read_unlock();
 			if (delegation != NULL) {
-				delegation = nfs_detach_delegation(NFS_I(inode),
-					delegation, server);
-				if (delegation != NULL)
+				if (nfs_detach_delegation(NFS_I(inode), delegation,
+							server) != NULL)
 					nfs_free_delegation(delegation);
+				/* Match nfs_start_delegation_return_locked */
+				nfs_put_delegation(delegation);
 			}
 			iput(inode);
 			nfs_sb_deactive(server->super);
diff --git a/fs/nfs/delegation.h b/fs/nfs/delegation.h
index 31b8460..9b00a0b 100644
--- a/fs/nfs/delegation.h
+++ b/fs/nfs/delegation.h
@@ -22,6 +22,7 @@
 	unsigned long pagemod_limit;
 	__u64 change_attr;
 	unsigned long flags;
+	refcount_t refcount;
 	spinlock_t lock;
 	struct rcu_head rcu;
 };