MTX Lock implementation question

Suppose the following:

Module foo.c

foo_getlock(sc)
{
   FOO_LOCK(sc);
}
foo_unlock(sc)
{
   FOO_UNLOCK(sc);
}
foo_dosomething(sc)
{
   MTX_LOCK_ASSERT(sc);
   foo_dooit();
}

Module bar.c

bar_dofoo()
{
   foo_getlock(sc);
   foo_dosomething(sc);
   foo_unlock(sc);
}


Is this something that shouldn't work? I need to access functions
 that require locks in a different module, but this code barfs on 
ASSERT with witness enabled. Is this a deflugalty in WITNESS, or do 
I need to create functions within foo that do the locking? Its been
working ok for awhile (its not a high volume function) but when I 
fired up witness to debug something else I encountered witness panics.

Thanks,

Barney