Subject: Re: [patch] mutex: optimise generic mutex implementations

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 23 Oct 2008 15:43:58 +1100

On Wed, 2008-10-22 at 17:59 +0200, Ingo Molnar wrote:
> * Nick Piggin <[email-masked]> wrote:
> 
> > Speed up generic mutex implementations.
> > 
> > - atomic operations which both modify the variable and return something imply
> >   full smp memory barriers before and after the memory operations involved
> >   (failing atomic_cmpxchg, atomic_add_unless, etc don't imply a barrier because
> >   they don't modify the target). See Documentation/atomic_ops.txt.
> >   So remove extra barriers and branches.
> >   
> > - All architectures support atomic_cmpxchg. This has no relation to
> >   __HAVE_ARCH_CMPXCHG. We can just take the atomic_cmpxchg path unconditionally
> > 
> > This reduces a simple single threaded fastpath lock+unlock test from 590 cycles
> > to 203 cycles on a ppc970 system.
> > 
> > Signed-off-by: Nick Piggin <[email-masked]>
> 
> no objections here. Lets merge these two patches via the ppc tree, so 
> that it gets testing on real hardware as well?
> 
> Acked-by: Ingo Molnar <[email-masked]>

Allright but in that case it will be after -rc1 unless I manage to sneak
something in tomorrow before linux closes the merge window.

I can't get an update today.

Cheers,
Ben.