nfs: add memory barriers around NFS_INO_INVALID_DATA and NFS_INO_INVALIDATING

If the setting of NFS_INO_INVALIDATING gets reordered to before the
clearing of NFS_INO_INVALID_DATA, then another task may hit a race
window where both appear to be clear, even though the inode's pages are
still in need of invalidation. Fix this by adding the appropriate memory
barriers.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/inode.c

@@ -1050,6 +1050,7 @@ int nfs_revalidate_mapping(struct inode *inode, struct address_space *mapping)
 	}
 
 	set_bit(NFS_INO_INVALIDATING, bitlock);
+	smp_wmb();
 	nfsi->cache_validity &= ~NFS_INO_INVALID_DATA;
 	spin_unlock(&inode->i_lock);
 	trace_nfs_invalidate_mapping_enter(inode);