scst_dlm: Warn if kernel_read() can't read from configfs files

See also upstream commit 7fe1e79b59ba ("configfs: implement the .read_iter
and .write_iter methods"; v5.14).



git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@9594 d57e44dd-8a1f-0410-8b47-8ef2f437770f

scst/src/scst_dlm.c

@@ -510,6 +510,13 @@ static int scst_read_file(const char *path, char *buf, int buf_len)
 		goto out;
 	}
 	pos = 0;
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 10, 0)
+	/*
+	 * Commit 4d03e3cc5982 ("fs: don't allow kernel reads and writes
+	 * without iter ops") made kernel_read() depend on .read_iter.
+	 */
+	WARN_ON_ONCE(!f->f_op->read_iter);
+#endif
 	ret = kernel_read(f, buf, buf_len, &pos);
 	if (ret >= 0)
 		buf[min(ret, buf_len - 1)] = '\0';