Re: 2.4.x: i386/x86_64 bitops clobberings

[Willy Tarreau <w@1wt.eu>]

On Mon, Oct 09, 2006 at 12:53:56AM +0200, Jan Kiszka wrote:
> Willy Tarreau wrote:
> > Hi Jan,
> > 
> > On Sun, Oct 08, 2006 at 11:42:07PM +0200, Jan Kiszka wrote:
> >> Hi,
> >>
> >> after going through debugging hell with some out-of-tree code, I
> >> realised that this patch
> >>
> >> http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=92934bcbf96bc9dc931c40ca5f1a57685b7b813b
> >>
> >> makes a difference: current 2.6 works with the following code sequence
> >> as expected (printk is executed), 2.4 fails.
> >>
> >>
> >> #include <asm/bitops.h>
> >> #include <linux/module.h>
> >>
> >> unsigned long a = 1;
> >>
> >> int module_init(void)
> >> {
> >> 	unsigned long b = 0;
> >> 	int x;
> >>
> >> 	x = __test_and_set_bit(0, &b);
> >> 	if (__test_and_set_bit(0, &a))
> >> 		printk("x = %d\n", x);
> >>
> >> 	return -1;
> >> }
> >>
> >>
> >> There will likely be a way to work around my issue. Nevertheless, I
> >> wondered if that patch was already considered for 2.4 inclusion. Or is
> >> there no risk that in-tree code is affected?
> > 
> > While I remember some discussion on the subject for 2.6, I don't
> > recall anything similiar in 2.4. Wouldn't you happen to build with
> > gcc-3.4 ? IIRC, the clobbering changed around this version. Could
> 
> I used gcc 3.3.6-13 (Debian).

OK, fine.

> > you confirm that the patch you pointed above fixes the problem for
> > your case ?
> 
> Yes. I ported the x86 part back to 2.4.33 and my problem disappeared.

I've done it too, but unfortunately, I discovered that it does not build
with gcc-2.95 anymore, while 3.3 is fine :

cyclades.c: In function `cyy_interrupt':
/usr/src/git/linux-2.4/include/asm/bitops.h:130: inconsistent operand constraints in an `asm'
cyclades.c: In function `cyz_handle_rx':
/usr/src/git/linux-2.4/include/asm/bitops.h:130: inconsistent operand constraints in an `asm'

127        __asm__ __volatile__( LOCK_PREFIX
128                "btsl %2,%1\n\tsbbl %0,%0"
129                :"=r" (oldbit),"+m" (ADDR)
130                :"Ir" (nr) : "memory");


I don't know what the right solution should be. I'm not fond of #ifdefs,
and I'm embarrassed to know that gcc can do all sorts of nasty things due
to a wrong clobbering :-/

If you have any idea on the subject, I'm willing to try.

Regards,
Willy