Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

Bernd Jendrissek
2003-05-30T18:30:48Z

On Fri, May 30, 2003 at 10:33:29AM -0700, Joe Buck wrote:
> On Fri, May 30, 2003 at 07:22:40PM +0200, Bernd Jendrissek wrote:
>
> > If you look at linux/include/linux/spinlock.h, you'll see:
> >
> > /*
> > * Your basic spinlocks, allowing only a single CPU anywhere
> > *
> > * Most gcc versions have a nasty bug with empty initializers.
> > */
> > #if (__GNUC__ > 2)
> > typedef struct { } spinlock_t;
> > #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
> > #else
> > typedef struct { int gcc_is_buggy; } spinlock_t;
> > #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
> > #endif
>
> Yuk! What is the benefit of introducing this incompatibility? #ifdefs
> are harmful to maintainance, and it's only one word, so why not always
> put in the dummy struct member?

I dont speak for the kernel people, but...

I suppose some people just insist on squeezing every last cycle out of
their machines. For my home PC (a 486 with 5MB RAM running linux 2.0.30),
I am quite grateful for such cycle and bit saving. Believe me, I notice
whether I have apache running or not. :)

Hmm, yes, it does seem to be just one word. grep -r spinlock_t . |wc -l
says 1013 here, that's across *all* architectures. IOW 4052 bytes - that's
*one page* - on i386!

Never mind what definition tcc will give to __GNUC__

So there I thought I was going to justify the kernel. Instead I mostly
agree with Joe! I'm also sure there have been flamewars about this...

> > Hmm, actually I thought the kernel had a mechanism to prevent a GCC 3.x
> > module from being loaded into a GCC 2.x kernel and vice versa?
>
> Is there any reason, other than the above-described bit of evil, for doing
> this (forbidding mixing)? It prevents the bug-finding approach I
> described earlier (a binary search for finding miscompiled code) from
> working.

Between GCC 2.x and 3.x the *major* version changed (duh). I would
imagine that people are/were (justifiably?) concerned that ABI's might
have changed. From your response, I assume there are no ABI changes
for C at least? I suppose a gratuitous ABI change would constitute a
bug, though...

BTW I said "I thought" - it appears there is in fact no such mechanism.

Okay, so here's a PR (Public Relations, not Problem Report) patch just
for you, Joe: <with a fistful of smileys :)>

(It also gets rid of some of that crazy 2-space indentation.)

diff -u linux/include/linux/spinlock.h.borig linux/include/linux/spinlock.h
--- linux/include/linux/spinlock.h.borig Tue May 13 17:05:57 2003
+++ linux/include/linux/spinlock.h Fri May 30 20:29:42 2003
@@ -53,13 +53,8 @@
*
* Most gcc versions have a nasty bug with empty initializers.
*/
-#if (__GNUC__ > 2)
- typedef struct { } spinlock_t;
- #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
-#else
- typedef struct { int gcc_is_buggy; } spinlock_t;
- #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
-#endif
+typedef struct { int gcc_was_buggy; } spinlock_t;
+#define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }

#define spin_lock_init(lock) do { } while(0)
#define spin_lock(lock) (void)(lock) /* Not "unused variable". */