sx_xlocked(9) on FreeBSD
The text c0d639da6bb5, as FreeBSD 15.0-RELEASE, FreeBSD 14.5-RELEASE, FreeBSD 14.4-RELEASE, FreeBSD 14.3-RELEASE, FreeBSD 14.2-RELEASE, FreeBSD 14.1-RELEASE, FreeBSD 14.0-RELEASE, FreeBSD 13.5-RELEASE, FreeBSD 13.4-RELEASE, FreeBSD 13.3-RELEASE shipped it.
kernel shared/exclusive lock