loop/010: do not assume /dev/loop0

The current implementation of the test case loop/010 assumes that the
prepared loop device is /dev/loop0, which is not always true. When other
loop devices are set up before the test case run, the assumption is
wrong and the test case fails.

To avoid the failure, use the prepared loop device name stored in
$loop_device instead of /dev/loop0. Adjust the grep string to meet the
device name. Also use "losetup --detach" instead of
"losetup --detach-all" to not detach the loop devices which existed
before the test case runs.

Fixes: 1c4ae4f ("loop: Detect a race condition between loop detach and open")
Reviewed-by: Gulam Mohamed <gulam.mohamed@oracle.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>

tests/loop/010

@@ -16,18 +16,23 @@ requires() {
 }
 
 create_loop() {
+	local dev=$1
+
 	while true
 	do
-		loop_device="$(losetup --partscan --find --show "${image_file}")"
-		blkid /dev/loop0p1 >& /dev/null
+		if losetup --partscan "$dev" "${image_file}" &> /dev/null; then
+			blkid "$dev"p1 >& /dev/null
+		fi
 	done
 }
 
 detach_loop() {
+	local dev=$1
+
 	while true
 	do
-		if [ -e /dev/loop0 ]; then
-			losetup --detach /dev/loop0 >& /dev/null
+		if [[ -e "$dev" ]]; then
+			losetup --detach "$dev" >& /dev/null
 		fi
 	done
 }
@@ -38,6 +43,7 @@ test() {
 	local create_pid
 	local detach_pid
 	local image_file="$TMPDIR/loopImg"
+	local grep_str
 
 	truncate --size 1G "${image_file}"
 	parted --align none --script "${image_file}" mklabel gpt
@@ -53,9 +59,9 @@ test() {
 	mkfs.xfs --force "${loop_device}p1" >& /dev/null
 	losetup --detach "${loop_device}" >&  /dev/null
 
-	create_loop &
+	create_loop "${loop_device}" &
 	create_pid=$!
-	detach_loop &
+	detach_loop "${loop_device}" &
 	detach_pid=$!
 
 	sleep "${TIMEOUT:-90}"
@@ -66,8 +72,9 @@ test() {
 		sleep 1
 	} 2>/dev/null
 
-	losetup --detach-all >& /dev/null
-	if _dmesg_since_test_start | grep --quiet "partition scan of loop0 failed (rc=-16)"; then
+	losetup --detach "${loop_device}" >& /dev/null
+	grep_str="partition scan of ${loop_device##*/} failed (rc=-16)"
+	if _dmesg_since_test_start | grep --quiet "$grep_str"; then
 		echo "Fail"
 	fi
 	echo "Test complete"