selftests: ublk: make sure _add_ublk_dev can return in sub-shell

Detach ublk daemon from the starting process completely by double-fork and
clearing its process group, so that `_add_ublk_dev` can return from sub-shell.

Then it is more friendly for writing shell test script for adding/recovering
ublk device.

Prepare for running ublk test in parallel.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-5-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

tools/testing/selftests/ublk/kublk.c

@@ -654,6 +654,8 @@ static int ublk_send_dev_event(const struct dev_ctx *ctx, int dev_id)
 	if (write(evtfd, &id, sizeof(id)) != sizeof(id))
 		return -EINVAL;
 
+	close(evtfd);
+
 	return 0;
 }
 
@@ -889,24 +891,40 @@ static int cmd_dev_add(struct dev_ctx *ctx)
 		exit(-1);
 	}
 
-	setsid();
 	res = fork();
 	if (res == 0) {
+		int res2;
+
+		setsid();
+		res2 = fork();
+		if (res2 == 0) {
+			/* prepare for detaching */
+			close(STDIN_FILENO);
+			close(STDOUT_FILENO);
+			close(STDERR_FILENO);
 run:
-		res = __cmd_dev_add(ctx);
-		return res;
+			res = __cmd_dev_add(ctx);
+			return res;
+		} else {
+			/* detached from the foreground task */
+			exit(EXIT_SUCCESS);
+		}
 	} else if (res > 0) {
 		uint64_t id;
+		int exit_code = EXIT_FAILURE;
 
 		res = read(ctx->_evtfd, &id, sizeof(id));
 		close(ctx->_evtfd);
 		if (res == sizeof(id) && id != ERROR_EVTFD_DEVID) {
 			ctx->dev_id = id - 1;
-			return __cmd_dev_list(ctx);
+			if (__cmd_dev_list(ctx) >= 0)
+				exit_code = EXIT_SUCCESS;
 		}
-		exit(EXIT_FAILURE);
+		/* wait for child and detach from it */
+		wait(NULL);
+		exit(exit_code);
 	} else {
-		return res;
+		exit(EXIT_FAILURE);
 	}
 }
 

tools/testing/selftests/ublk/test_common.sh

@@ -170,7 +170,6 @@ _have_feature()
 }
 
 _add_ublk_dev() {
-	local kublk_temp;
 	local dev_id;
 
 	if [ ! -c /dev/ublk-control ]; then
@@ -182,17 +181,17 @@ _add_ublk_dev() {
 		fi
 	fi
 
-	kublk_temp=$(mktemp /tmp/kublk-XXXXXX)
-	if ! "${UBLK_PROG}" add "$@" > "${kublk_temp}" 2>&1; then
+	if ! dev_id=$("${UBLK_PROG}" add "$@" | grep "dev id" | awk -F '[ :]' '{print $3}'); then
 		echo "fail to add ublk dev $*"
-		rm -f "${kublk_temp}"
 		return 255
 	fi
-
-	dev_id=$(grep "dev id" "${kublk_temp}" | awk -F '[ :]' '{print $3}')
 	udevadm settle
-	rm -f "${kublk_temp}"
-	echo "${dev_id}"
+
+	if [[ "$dev_id" =~ ^[0-9]+$ ]]; then
+		echo "${dev_id}"
+	else
+		return 255
+	fi
 }
 
 # kill the ublk daemon and return ublk device state

tools/testing/selftests/ublk/test_stress_01.sh

@@ -4,19 +4,19 @@
 . "$(cd "$(dirname "$0")" && pwd)"/test_common.sh
 TID="stress_01"
 ERR_CODE=0
-DEV_ID=-1
 
 ublk_io_and_remove()
 {
 	local size=$1
+	local dev_id
 	shift 1
 
-	DEV_ID=$(_add_ublk_dev "$@")
+	dev_id=$(_add_ublk_dev "$@")
 	_check_add_dev $TID $?
 
 	[ "$UBLK_TEST_QUIET" -eq 0 ] && echo "run ublk IO vs. remove device(ublk add $*)"
-	if ! __run_io_and_remove "${DEV_ID}" "${size}" "no"; then
-		echo "/dev/ublkc${DEV_ID} isn't removed"
+	if ! __run_io_and_remove "$dev_id" "${size}" "no"; then
+		echo "/dev/ublkc$dev_id isn't removed"
 		exit 255
 	fi
 }

tools/testing/selftests/ublk/test_stress_02.sh

@@ -4,19 +4,19 @@
 . "$(cd "$(dirname "$0")" && pwd)"/test_common.sh
 TID="stress_02"
 ERR_CODE=0
-DEV_ID=-1
 
 ublk_io_and_kill_daemon()
 {
 	local size=$1
+	local dev_id
 	shift 1
 
-	DEV_ID=$(_add_ublk_dev "$@")
+	dev_id=$(_add_ublk_dev "$@")
 	_check_add_dev $TID $?
 
 	[ "$UBLK_TEST_QUIET" -eq 0 ] && echo "run ublk IO vs kill ublk server(ublk add $*)"
-	if ! __run_io_and_remove "${DEV_ID}" "${size}" "yes"; then
-		echo "/dev/ublkc${DEV_ID} isn't removed res ${res}"
+	if ! __run_io_and_remove "$dev_id" "${size}" "yes"; then
+		echo "/dev/ublkc$dev_id isn't removed res ${res}"
 		exit 255
 	fi
 }