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

[Ingo Molnar]

* 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:

repeat:
       ldrex   r1, [r0]
       sub     r1, r1, #1
       strex   r2, r1, [r0]
       orrs    r0, r2, r1
       jneq    repeat

(shot-in-the-dark guess at ARMv6 assembly)

hm?

	Ingo
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