A well tool system for performing an operation in an oil/gas well, wherein the well tool system comprises: a well tool; a bore comprising a lock groove; wherein the well tool comprises: - a mandrel having a longitudinal direction (I-I) and a radial direction perpendicular to the longitudinal direction ( I-I); and - an anchoring device for longitudinally locking the well tool to the lock groove of the bore; wherein the anchoring device comprises: - a first ring arranged radially outside the mandrel; - a second ring arranged radially outside the mandrel at a longitudinal distance from the first ring and movable in the longitudinal direction ( I-I) relative to the first ring; - a spring element being substantially ring-shaped and arranged radially outside the mandrel; and - a plurality of locking dogs distributed circumferentially on the spring element and located longitudinally between the first ring and the second ring, the plurality of locking dogs being designed to be engaged into the lock groove of the bore; wherein the anchoring device has a run state in which the first ring and the second ring are provided at an initial longitudinal distance ( D R) from each other; and wherein the anchoring device has a set state in which the first ring and the second ring are provided at a shorter distance ( D S) from each other, thereby wedging the locking dogs outwards in the radial direction; wherein the spring element is configured to bias the locking dogs inwards in the radial direction towards the run state of the anchoring device. a well tool; a bore comprising a lock groove; wherein the well tool comprises: - a mandrel having a longitudinal direction (I-I) and a radial direction perpendicular to the longitudinal direction ( I-I); and - an anchoring device for longitudinally locking the well tool to the lock groove of the bore; - a mandrel having a longitudinal direction (I-I) and a radial direction perpendicular to the longitudinal direction ( I-I); and - an anchoring device for longitudinally locking the well tool to the lock groove of the bore; wherein the anchoring device comprises: - a first ring arranged radially outside the mandrel; - a second ring arranged radially outside the mandrel at a longitudinal distance from the first ring and movable in the longitudinal direction ( I-I) relative to the first ring; - a spring element being substantially ring-shaped and arranged radially outside the mandrel; and - a plurality of locking dogs distributed circumferentially on the spring element and located longitudinally between the first ring and the second ring, the plurality of locking dogs being designed to be engaged into the lock groove of the bore; - a first ring arranged radially outside the mandrel; - a second ring arranged radially outside the mandrel at a longitudinal distance from the first ring and movable in the longitudinal direction ( I-I) relative to the first ring; - a spring element being substantially ring-shaped and arranged radially outside the mandrel; and - a plurality of locking dogs distributed circumferentially on the spring element and located longitudinally between the first ring and the second ring, the plurality of locking dogs being designed to be engaged into the lock groove of the bore; wherein the anchoring device has a run state in which the first ring and the second ring are provided at an initial longitudinal distance ( D R) from each other; and wherein the anchoring device has a set state in which the first ring and the second ring are provided at a shorter distance ( D S) from each other, thereby wedging the locking dogs outwards in the radial direction; wherein the spring element is configured to bias the locking dogs inwards in the radial direction towards the run state of the anchoring device.