there's something i've been curious about for a while regarding
semaphores.
in robert love's kernel book (2005, so it's a bit out of date and
that might be an issue here), he's writing about semaphores and says
that statically declared semaphores can be declared with:
static DECLARE_SEMAPHORE_GENERIC(name, count);
sounds reasonable, but if you look in include/asm-i386/semaphore.h
these days, what you see is:
#define __DECLARE_SEMAPHORE_GENERIC(name,count) \
struct semaphore name = __SEMAPHORE_INITIALIZER(name,count)
so that macro has clearly been renamed, and *appears* to be treated
as the fundamental, underlying mechanism for creating a new semaphore.
in fact, it's used by:
#define DECLARE_MUTEX(name) __DECLARE_SEMAPHORE_GENERIC(name,1)
#define DECLARE_MUTEX_LOCKED(name) __DECLARE_SEMAPHORE_GENERIC(name,0)
it's as if, as a kernel programmer, you're not really supposed to call
__DECLARE_SEMAPHORE_GENERIC() directly anymore and, in fact, no one
does. everyone seems to declare an actual "struct semaphore", and use
"sema_init()" to initialize it. ok, fair enough. but look at the
case of read-write semaphores now.
if you look at include/asm-i386/rwsem.h, it has the corresponding
macro:
#define DECLARE_RWSEM(name) \
struct rw_semaphore name = __RWSEM_INITIALIZER(name)
and if you do a search:
$ grep -rw DECLARE_RWSEM .
you can see that people *are* invoking that directly fairly
frequently.
so why the different treatment? apparently, if it comes to
declaring a regular semaphore with something like DECLARE_SEMAPHORE,
that's just not done. but invoking DECLARE_RWSEM appears to be fine.
why the apparent inconsistency?
rday
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA
http://www.fsdev.dreamhosters.com/wiki/index.php?title=Main_Page
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