# PoC build (in-guest, root@dfbsd, DragonFly 6.5-DEVELOPMENT #1 X86_64_QUOTA, gcc 8.3)
$ cc -O -pthread -o /root/qt_race /root/qt_race.c
(no errors; one benign -Wdiscarded-qualifiers warning on the quotactl prototype)

# Kernel builds used for verification (make nativekernel, in-guest):
# baseline: cp sys/config/X86_64_GENERIC sys/config/X86_64_QUOTA && echo 'options QUOTA' >>; 
#   make nativekernel KERNCONF=X86_64_QUOTA && make installkernel KERNCONF=X86_64_QUOTA  (0 errors)
# fixed: same + fix.diff applied to sys/vfs/ufs/{ufs_quota.c,ufsmount.h,ffs_vfsops.c}
#   (0 errors, final build FIXBUILD2_DONE)