Re: [Ksummit-discuss] [TECH TOPIC] asm-generic implementations of low-level synchronisation constructs

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 07, 2014 at 05:17:48PM -0400, Waiman Long wrote:
> On 05/07/2014 02:29 PM, Will Deacon wrote:
> >Hi all,
> >
> >Traditionally, low-level synchronisation and atomic constructs have been
> >buried away in arch-specific code, with new arch maintainers having to
> >wrestle with Documentation/{memory-barriers,atomic_ops}.txt to ensure
> >they provide the (somewhat arbitrary) semantics expected by the kernel.
> >
> >However, over the past year, there have been some notable events in this
> >area:
> >
> >   (1) The addition of smp_load_acquire/smp_store_release across all
> >       architectures (including asm-generic)
> >       https://lwn.net/Articles/576486/
> >
> >   (2) Merging of generic MCS spinlocks into kernel/locking, built using
> >       the macros introduced by (1). There are other similar patches for
> >       queued spinlocks and rwlocks, but they're not completely generic
> >       afaict.
> >       http://lwn.net/Articles/590243/
> 
> It is true that the current qspinlock patch is not completely generic.
> However, I think it can still be used by most architectures with the
> exception of, perhaps just, the pre-EV56 alpha.

Its going to be a massive pain on virt archs like PPC and s390. So while
those archs don't suffer the same problem Alpha does, they have problem
with fair locks.