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 <[email protected]>
Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1071501
Fixes: 3c59366 ("NFS: don't unhash dentry during unlink/rename")
Signed-off-by: NeilBrown <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>

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;
  }