NFS: add barriers when testing for NFS_FSDATA_BLOCKED

dentry->d_fsdata is set to NFS_FSDATA_BLOCKED while unlinking or
renaming-over a file to ensure that no open succeeds while the NFS
operation progressed on the server.

Setting dentry->d_fsdata to NFS_FSDATA_BLOCKED is done under ->d_lock
after checking the refcount is not elevated.  Any attempt to open the
file (through that name) will go through lookp_open() which will take
->d_lock while incrementing the refcount, we can be sure that once the
new value is set, __nfs_lookup_revalidate() *will* see the new value and
will block.

We don't have any locking guarantee that when we set ->d_fsdata to NULL,
the wait_var_event() in __nfs_lookup_revalidate() will notice.
wait/wake primitives do NOT provide barriers to guarantee order.  We
must use smp_load_acquire() in wait_var_event() to ensure we look at an
up-to-date value, and must use smp_store_release() before wake_up_var().

This patch adds those barrier functions and factors out
block_revalidate() and unblock_revalidate() far clarity.

There is also a hypothetical bug in that if memory allocation fails
(which never happens in practice) we might leave ->d_fsdata locked.
This patch adds the missing call to unblock_revalidate().
Reported-and-tested-by: Richard Kojedzinszky <richard+debian+bugreport@kojedz.in>
Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1071501
Fixes: 3c59366c ("NFS: don't unhash dentry during unlink/rename")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/dir.c

@@ -1803,9 +1803,10 @@ __nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags,
 		if (parent != READ_ONCE(dentry->d_parent))
 			return -ECHILD;
 	} else {
-		/* Wait for unlink to complete */
+		/* Wait for unlink to complete - see unblock_revalidate() */
 		wait_var_event(&dentry->d_fsdata,
-			       dentry->d_fsdata != NFS_FSDATA_BLOCKED);
+			       smp_load_acquire(&dentry->d_fsdata)
+			       != NFS_FSDATA_BLOCKED);
 		parent = dget_parent(dentry);
 		ret = reval(d_inode(parent), dentry, flags);
 		dput(parent);
@@ -1818,6 +1819,29 @@ static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 	return __nfs_lookup_revalidate(dentry, flags, nfs_do_lookup_revalidate);
 }
 
+static void block_revalidate(struct dentry *dentry)
+{
+	/* old devname - just in case */
+	kfree(dentry->d_fsdata);
+
+	/* Any new reference that could lead to an open
+	 * will take ->d_lock in lookup_open() -> d_lookup().
+	 * Holding this lock ensures we cannot race with
+	 * __nfs_lookup_revalidate() and removes and need
+	 * for further barriers.
+	 */
+	lockdep_assert_held(&dentry->d_lock);
+
+	dentry->d_fsdata = NFS_FSDATA_BLOCKED;
+}
+
+static void unblock_revalidate(struct dentry *dentry)
+{
+	/* store_release ensures wait_var_event() sees the update */
+	smp_store_release(&dentry->d_fsdata, NULL);
+	wake_up_var(&dentry->d_fsdata);
+}
+
 /*
  * A weaker form of d_revalidate for revalidating just the d_inode(dentry)
  * when we don't really care about the dentry name. This is called when a
@@ -2551,15 +2575,12 @@ int nfs_unlink(struct inode *dir, struct dentry *dentry)
 		spin_unlock(&dentry->d_lock);
 		goto out;
 	}
-	/* old devname */
-	kfree(dentry->d_fsdata);
-	dentry->d_fsdata = NFS_FSDATA_BLOCKED;
+	block_revalidate(dentry);
 
 	spin_unlock(&dentry->d_lock);
 	error = nfs_safe_remove(dentry);
 	nfs_dentry_remove_handle_error(dir, dentry, error);
-	dentry->d_fsdata = NULL;
-	wake_up_var(&dentry->d_fsdata);
+	unblock_revalidate(dentry);
 out:
 	trace_nfs_unlink_exit(dir, dentry, error);
 	return error;
@@ -2666,8 +2687,7 @@ nfs_unblock_rename(struct rpc_task *task, struct nfs_renamedata *data)
 {
 	struct dentry *new_dentry = data->new_dentry;
 
-	new_dentry->d_fsdata = NULL;
-	wake_up_var(&new_dentry->d_fsdata);
+	unblock_revalidate(new_dentry);
 }
 
 /*
@@ -2729,11 +2749,6 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 		if (WARN_ON(new_dentry->d_flags & DCACHE_NFSFS_RENAMED) ||
 		    WARN_ON(new_dentry->d_fsdata == NFS_FSDATA_BLOCKED))
 			goto out;
-		if (new_dentry->d_fsdata) {
-			/* old devname */
-			kfree(new_dentry->d_fsdata);
-			new_dentry->d_fsdata = NULL;
-		}
 
 		spin_lock(&new_dentry->d_lock);
 		if (d_count(new_dentry) > 2) {
@@ -2755,7 +2770,7 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 			new_dentry = dentry;
 			new_inode = NULL;
 		} else {
-			new_dentry->d_fsdata = NFS_FSDATA_BLOCKED;
+			block_revalidate(new_dentry);
 			must_unblock = true;
 			spin_unlock(&new_dentry->d_lock);
 		}
@@ -2767,6 +2782,8 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 	task = nfs_async_rename(old_dir, new_dir, old_dentry, new_dentry,
 				must_unblock ? nfs_unblock_rename : NULL);
 	if (IS_ERR(task)) {
+		if (must_unblock)
+			unblock_revalidate(new_dentry);
 		error = PTR_ERR(task);
 		goto out;
 	}