linux-kernel - Re: [RFC PATCH v3] locking/atomic: Implement atomic{,64,_long}_{fetch_,}{andnot_or}{,_relaxed,_acquire,

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Date:   Thu, 29 Jul 2021 13:35:23 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Rui Wang <wangrui@...ngson.cn>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, Guo Ren <guoren@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rui Wang <r@....cc>, Xuefeng Li <lixuefeng@...ngson.cn>,
        Huacai Chen <chenhuacai@...il.com>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        kernel test robot <lkp@...el.com>
Subject: Re: [RFC PATCH v3] locking/atomic: Implement
 atomic{,64,_long}_{fetch_,}{andnot_or}{,_relaxed,_acquire,_release}()

On Thu, Jul 29, 2021 at 01:43:41PM +0200, Arnd Bergmann wrote:
> On Thu, Jul 29, 2021 at 11:56 AM Will Deacon <will@...nel.org> wrote:
> > On Thu, Jul 29, 2021 at 05:30:03PM +0800, Rui Wang wrote:
> > > This patch introduce a new atomic primitive andnot_or:
> >
> > Please see my other comments on the other patches you posted:
> >
> > https://lore.kernel.org/r/20210729093923.GD21151@willie-the-truck
> >
> > Overall, I'm not thrilled to bits by extending the atomics API with
> > operations that cannot be implemented efficiently on any (?) architectures
> > and are only used by the qspinlock slowpath on machines with more than 16K
> > CPUs.
> 
> Wouldn't this also help improve set_mask_bits()? That one at least has
> a handful of users in the kernel.

For pure LL/SC architectures, yes, but I don't think it helps anybody else.

Afaict, an architecture can already override set_mask_bits, so why do we
need to add this primitive to the atomic API?

Will