Re: [patch] spinlocks: remove 'volatile'

[Vadim Lobanov]

On Sat, 8 Jul 2006, J.A. [UTF-8] Magallón wrote:

> On Fri, 7 Jul 2006 17:22:22 -0500 (CDT), Chase Venters <chase.venters@xxxxxxxxxxxx> wrote:
>
> > > .L7:
> > >    movl    $1, mtx    <=========
> > >    movl    spinvar, %eax
> > >    movl    $0, mtx    <=========
> > >    testl   %eax, %eax
> > >    jne .L7
> > >    popl    %ebp
> > >    ret
> >
> > NO! It's not better. You're still not syncing or locking the bus! If you
> > refer to the fact that the "movl $1" has magically appeared, that's
> > because you've just PAPERED OVER THE PROBLEM WITH "volatile", which is
> > _exactly_ what Linus is telling you NOT TO DO.
> >
>
> BTW, I really don't mind if a given architecnture has to lock the bus or
> say a prayer to Budha to reload a variable. I want it to be reloaded at
> every (or a certain, in case of a (volatile)mtx cast) usage. The compiler
> is the responsible of knowing what to do. What if nextgen P4 Xeon do not
> need a bus lock ? Will you rewrite the kernel ?

Looks like you need to read Documentation/memory-barriers.txt. That file
explains why the above assembly code is not correct.

Bonus question: what stops the processor from coalescing or rearranging
the three movl instructions in that assembly?

> --
> J.A. Magallon <jamagallon()ono!com>     \               Software is like sex:
>                                          \         It's better when it's free
> Mandriva Linux release 2007.0 (Cooker) for i586
> Linux 2.6.17-jam01 (gcc 4.1.1 20060518 (prerelease)) #2 SMP PREEMPT Wed

-- Vadim Lobanov
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