which lock protects sysctl instances ?

hi,
is there any protection in RELENG_5 and above for multiple
sysctl handlers ?

In other words, assume i have
SYSCTL_PROC(_kern, OID_AUTO, foo, CTLTYPE_STRING|CTLFLAG_RW,
       0, 0, sysctl_foo_handler, "A", "bla bla bla");

do i have to worry about multiple instances of sysctl_foo_handler()
running in parallel, or they run under Giant, or some other lock ?

	thanks
	luigi