Re: [patch] spinlocks: remove 'volatile'

Vadim Lobanov
2006-07-07T22:59:36Z

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: TEXT/PLAIN; charset=X-UNKNOWN, Size: 1535 bytes --]

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@clientec.com> 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