Index: uspace/srv/vfs/vfs_ops.c
===================================================================
--- uspace/srv/vfs/vfs_ops.c	(revision f07f6b2eb8e7ca48fba796590394b94acc09b8bc)
+++ uspace/srv/vfs/vfs_ops.c	(revision b38dfd8028d6cec3849c0eccfe3dff997f218265)
@@ -781,4 +781,5 @@
 static void vfs_rdwr(ipc_callid_t rid, ipc_call_t *request, bool read)
 {
+	vfs_info_t *vi;
 
 	/*
@@ -807,9 +808,13 @@
 	fibril_mutex_lock(&file->lock);
 
+	vi = fs_handle_to_info(file->node->fs_handle);
+	assert(vi);
+
 	/*
 	 * Lock the file's node so that no other client can read/write to it at
-	 * the same time.
-	 */
-	if (read)
+	 * the same time unless the FS supports concurrent reads/writes and its
+	 * write implementation does not modify the file size.
+	 */
+	if (read || (vi->concurrent_read_write && vi->write_retains_size))
 		fibril_rwlock_read_lock(&file->node->contents_rwlock);
 	else
@@ -857,5 +862,5 @@
 	
 	/* Unlock the VFS node. */
-	if (read)
+	if (read || (vi->concurrent_read_write && vi->write_retains_size))
 		fibril_rwlock_read_unlock(&file->node->contents_rwlock);
 	else {