Re: [patch 04/15] Generic Mutex Subsystem, add-atomic-call-func-x86_64.patch

[Nick Piggin]

Nicolas Pitre wrote:
> On Tue, 20 Dec 2005, Ingo Molnar wrote:
>
>
> > * David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> >
> >
> > > On Mon, 2005-12-19 at 09:49 -0800, Zwane Mwaikambo wrote:
> > >
> > > > Hi Ingo,
> > > >        Doesn't this corrupt caller saved registers?
> > >
> > > Looks like it. I _really_ don't like calling functions from inline 
> > > asm. It's not nice. Can't we use atomic_dec_return() for this?
> >
> > we can use atomic_dec_return(), but that will add one more instruction 
> > to the fastpath. OTOH, atomic_dec_return() is available on every 
> > architecture, so it's a really tempting thing. I'll experiment with it.
>
>
> Please consider using (a variant of) xchg() instead.  Although 
> atomic_dec() is available on all architectures, its implementation is 
> far from being the most efficient thing to do for them all.  For 
> example, see my discussion about swp on ARM:

Considering that on UP, the arm should not need to disable interrupts
for this function (or has someone refuted Linus?), how about:

#ifndef CONFIG_SMP
typedef struct { volatile int counter; } mutex_counter_t;

static inline int mutex_counter_dec_return(mutex_counter *v)
{
    return --v->counter;
}

...
#else
#define mutex_counter_t atomic_t
...
#endif

Or does that get too bulky or have other problems?

MP ARMs should have an adequate atomic_dec_return.

--
SUSE Labs, Novell Inc.

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