Re: [patch 0/9] mutex subsystem, -V4

[Nicolas Pitre]

On Thu, 22 Dec 2005, Ingo Molnar wrote:

> 
> * Nicolas Pitre <nico@xxxxxxx> wrote:
> 
> > > i'm curious, how would this ARMv6 solution look like, and what would be 
> > > the advantages over the atomic swap based variant?
> > 
> > On ARMv6 (which can be SMP) the atomic swap instruction is much more 
> > costly than on former ARM versions.  It however has ll/sc instructions 
> > which allows it to implement a true atomic decrement, and the lock 
> > fast path would look like: [...]
> 
> but couldnt you implement atomic_dec_return() with the ll/sc 
> instructions? Something like:

NO.  My first example was atomic_dec_return based.  The second is 
lighter and fulfill the semantics of arch_mutex_fast_lock() but is not a 
common atomic primitive.


Nicolas
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