Re: x86 instructions reordering

[christos%tac.gw.com@localhost (Christos Zoulas)]

In article <20050324152233.GA18782%antioche.lip6.fr@localhost>,
Manuel Bouyer  <bouyer%antioche.lip6.fr@localhost> wrote:
>Hi,
>can newer x86 CPUs (hyperthreaded p4 in my case) reorder instructions,
>or memory writes ? If so, how can we impose barriers ? I didn't find
>anything obvious in the x86 SMP code, beside bit atomic operations (which
>don't work in my case).
>
>Basically I have these 2 pieces of code in xen (NetBSD and linux), one sender
>and one receiver, using a piece of shared memory.
>The receiver:                          |     The sender:
>                                       |
>handle_event()                         |     send()
>{                                      |     {
>                                       |             a = shared_memory->a;
>again:                                 |             do_something;
>        a = shared_memory->a;          |             wmb();
>        __insn_barrier();              |             shared_memory->a = a + 1;
>        b = shared_memory->b;          |             mb()
>        while (b < a) {                |             if (shared_memory->b == a)
>                /* do something */     |                     send_event();
>                did_something = 1;     |     }
>                b++;                   |
>        }                              |
>        __insn_barrier();              |
>        shared_memory->b = b;          |
>        __insn_barrier();              |
>        if (did_something)             |
>                goto again;            |
>}                                      |
>
>The sender is a piece of linux code, mb() and wmb() are both
>__asm__ __volatile__ ("lock; addl $0,0(%%esp)": : :"memory")
>which is the same as our x86_lfence(). I tried remplacing __insn_barrier
>with x86_lfence but the assembly produced by gcc didn't change.

It did not put:

#APP
        lock; addl $0, 0(%esp)
#NO_APP

???

It should have...

christos