:syn match cTodo '\<\i*[lL]ock\i*\>'
Reuses colors of the "TODO" and "XXX" items.
Edit1 I like this one better:
:syn match WarningMsg '\c\i*\(un\)\@<!\(lock\)\i*'
:syn match MoreMsg '\c\i*unlock\i*'
Red (WarningMsg) for lock, green (MoreMsg) for unlock. Trick is in the regexp which allows to match "lock" in all cases except "unlock". Packed into a function to be called when debugging locking.