Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 23, 2024 at 09:28:31AM -0700, Linus Torvalds wrote:
> On Wed, 18 Sept 2024 at 08:22, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Wed, 18 Sept 2024 at 13:15, Christoph Lameter (Ampere) <cl@gentwo.org> wrote:
> > >
> > > Other arches do not have acquire / release and will create additional
> > > barriers in the fallback implementation of smp_load_acquire. So it needs
> > > to be an arch config option.
> >
> > Actually, I looked at a few cases, and it doesn't really seem to be true.
> 
> Bah. I ended up just committing the minimal version of this all. I
> gave Christoph credit for the commit, because I stole his commit
> message, and he did most of the work, I just ended up going "simplify,
> simplify, simplify".
> 
> I doubt anybody will notice, and smp_load_acquire() is the future. Any
> architecture that does badly on it just doesn't matter (and, as
> mentioned, I don't think they even exist - "smp_rmb()" is generally at
> least as expensive).

Do we want to do the complementing patch and make write_seqcount_end()
use smp_store_release() ?

I think at least ARM (the 32bit thing) has wmb but uses mb for
store_release. But I also think I don't really care about that.