selftests/ublk: add test for async partition scan

Add test_generic_15.sh to verify that async partition scan prevents
IO hang when reading partition tables.

The test creates ublk devices with fault_inject target and very large
delay (60s) to simulate blocked partition table reads, then kills the
daemon to verify proper state transitions without hanging:

1. Without recovery support:
- Create device with fault_inject and 60s delay
- Kill daemon while partition scan may be blocked
- Verify device transitions to DEAD state

2. With recovery support (-r 1):
- Create device with fault_inject, 60s delay, and recovery
- Kill daemon while partition scan may be blocked
- Verify device transitions to QUIESCED state

Before the async partition scan fix, killing the daemon during
partition scan would cause deadlock as partition scan held ub->mutex
while waiting for IO. With the async fix, partition scan happens in
a work function and flush_work() ensures proper synchronization.

Add _add_ublk_dev_no_settle() helper function to skip udevadm settle,
which would otherwise hang waiting for partition scan events to
complete when partition table read is delayed.

Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

tools/testing/selftests/ublk/Makefile

···
 TEST_PROGS += test_generic_12.sh
 TEST_PROGS += test_generic_13.sh
 TEST_PROGS += test_generic_14.sh
+TEST_PROGS += test_generic_15.sh
 
 TEST_PROGS += test_null_01.sh
 TEST_PROGS += test_null_02.sh

tools/testing/selftests/ublk/test_common.sh

···
 _create_ublk_dev() {
 	local dev_id;
 	local cmd=$1
+	local settle=$2
 
-	shift 1
+	shift 2
 
 	if [ ! -c /dev/ublk-control ]; then
 		return ${UBLK_SKIP_CODE}
···
 		echo "fail to add ublk dev $*"
 		return 255
 	fi
-	udevadm settle
+
+	if [ "$settle" = "yes" ]; then
+		udevadm settle
+	fi
 
 	if [[ "$dev_id" =~ ^[0-9]+$ ]]; then
 		echo "${dev_id}"
···
 }
 
 _add_ublk_dev() {
-	_create_ublk_dev "add" "$@"
+	_create_ublk_dev "add" "yes" "$@"
+}
+
+_add_ublk_dev_no_settle() {
+	_create_ublk_dev "add" "no" "$@"
 }
 
 _recover_ublk_dev() {
 	local dev_id
 	local state
 
-	dev_id=$(_create_ublk_dev "recover" "$@")
+	dev_id=$(_create_ublk_dev "recover" "yes" "$@")
 	for ((j=0;j<20;j++)); do
 		state=$(_get_ublk_dev_state "${dev_id}")
 		[ "$state" == "LIVE" ] && break

tools/testing/selftests/ublk/test_generic_15.sh

···
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+. "$(cd "$(dirname "$0")" && pwd)"/test_common.sh
+
+TID="generic_15"
+ERR_CODE=0
+
+_test_partition_scan_no_hang()
+{
+	local recovery_flag=$1
+	local expected_state=$2
+	local dev_id
+	local state
+	local daemon_pid
+	local start_time
+	local elapsed
+
+	# Create ublk device with fault_inject target and very large delay
+	# to simulate hang during partition table read
+	# --delay_us 60000000 = 60 seconds delay
+	# Use _add_ublk_dev_no_settle to avoid udevadm settle hang waiting
+	# for partition scan events to complete
+	if [ "$recovery_flag" = "yes" ]; then
+		echo "Testing partition scan with recovery support..."
+		dev_id=$(_add_ublk_dev_no_settle -t fault_inject -q 1 -d 1 --delay_us 60000000 -r 1)
+	else
+		echo "Testing partition scan without recovery..."
+		dev_id=$(_add_ublk_dev_no_settle -t fault_inject -q 1 -d 1 --delay_us 60000000)
+	fi
+
+	_check_add_dev "$TID" $?
+
+	# The add command should return quickly because partition scan is async.
+	# Now sleep briefly to let the async partition scan work start and hit
+	# the delay in the fault_inject handler.
+	sleep 1
+
+	# Kill the ublk daemon while partition scan is potentially blocked
+	# And check state transitions properly
+	start_time=${SECONDS}
+	daemon_pid=$(_get_ublk_daemon_pid "${dev_id}")
+	state=$(__ublk_kill_daemon "${dev_id}" "${expected_state}")
+	elapsed=$((SECONDS - start_time))
+
+	# Verify the device transitioned to expected state
+	if [ "$state" != "${expected_state}" ]; then
+		echo "FAIL: Device state is $state, expected ${expected_state}"
+		ERR_CODE=255
+		${UBLK_PROG} del -n "${dev_id}" > /dev/null 2>&1
+		return
+	fi
+	echo "PASS: Device transitioned to ${expected_state} in ${elapsed}s without hanging"
+
+	# Clean up the device
+	${UBLK_PROG} del -n "${dev_id}" > /dev/null 2>&1
+}
+
+_prep_test "partition_scan" "verify async partition scan prevents IO hang"
+
+# Test 1: Without recovery support - should transition to DEAD
+_test_partition_scan_no_hang "no" "DEAD"
+
+# Test 2: With recovery support - should transition to QUIESCED
+_test_partition_scan_no_hang "yes" "QUIESCED"
+
+_cleanup_test "partition_scan"
+_show_result $TID $ERR_CODE