diff --git a/grep.c b/grep.c
index b29d09c7f..7a070e97c 100644
--- a/grep.c
+++ b/grep.c
@@ -948,8 +948,8 @@ int grep_threads_ok(const struct grep_opt *opt)
 	 * machinery in grep_buffer_1. The attribute code is not
 	 * thread safe, so we disable the use of threads.
 	 */
-	if (opt->funcname && !opt->unmatch_name_only && !opt->status_only &&
-	    !opt->name_only)
+	if ((opt->funcname || opt->funcbody)
+	    && !opt->unmatch_name_only && !opt->status_only && !opt->name_only)
 		return 0;
 
 	return 1;
@@ -1008,7 +1008,8 @@ static int grep_buffer_1(struct grep_opt *opt, const char *name,
 	}
 
 	memset(&xecfg, 0, sizeof(xecfg));
-	if (opt->funcname && !opt->unmatch_name_only && !opt->status_only &&
+	if ((opt->funcname || opt->funcbody)
+	    && !opt->unmatch_name_only && !opt->status_only &&
 	    !opt->name_only && !binary_match_only && !collect_hits) {
 		struct userdiff_driver *drv = userdiff_find_by_path(name);
 		if (drv && drv->funcname.pattern) {
diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
index 81263b785..7ba5b16f9 100755
--- a/t/t7810-grep.sh
+++ b/t/t7810-grep.sh
@@ -523,6 +523,20 @@ test_expect_success 'grep -W' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+hello.c=	printf("Hello world.\n");
+hello.c:	return 0;
+hello.c-	/* char ?? */
+EOF
+
+test_expect_success 'grep -W with userdiff' '
+	test_when_finished "rm -f .gitattributes" &&
+	git config diff.custom.xfuncname "(printf.*|})$" &&
+	echo "hello.c diff=custom" >.gitattributes &&
+	git grep -W return >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'grep from a subdirectory to search wider area (1)' '
 	mkdir -p s &&
 	(