linux-kernel - Re: [PATCH 2/5] riscv: Add QUEUED_SPINLOCKS & QUEUED

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Date:   Tue, 24 Nov 2020 16:00:14 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Guo Ren <guoren@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
        Michael Clark <michaeljclark@....com>
Subject: Re: [PATCH 2/5] riscv: Add QUEUED_SPINLOCKS & QUEUED_RWLOCKS supported

On Tue, Nov 24, 2020 at 3:39 PM Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Nov 24, 2020 at 01:43:54PM +0000, guoren@...nel.org wrote:
> > diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild

> > +             if (align) {                                            \
> > +             __asm__ __volatile__ (                                  \
> > +                     "0:     lr.w %0, 0(%z4)\n"                      \
> > +                     "       move %1, %0\n"                          \
> > +                     "       slli %1, %1, 16\n"                      \
> > +                     "       srli %1, %1, 16\n"                      \
> > +                     "       move %2, %z3\n"                         \
> > +                     "       slli %2, %2, 16\n"                      \
> > +                     "       or   %1, %2, %1\n"                      \
> > +                     "       sc.w %2, %1, 0(%z4)\n"                  \
> > +                     "       bnez %2, 0b\n"                          \
> > +                     "       srli %0, %0, 16\n"                      \
> > +                     : "=&r" (__ret), "=&r" (tmp), "=&r" (__rc)      \
> > +                     : "rJ" (__new), "rJ"(addr)                      \
> > +                     : "memory");                                    \
>
> I'm pretty sure there's a handfull of implementations like this out
> there... if only we could share.

Isn't this effectively the same as the "_Q_PENDING_BITS != 8"
version of xchg_tail()?

If nothing else needs xchg() on a 16-bit value, maybe
changing the #ifdef in the qspinlock code is enough.

Only around half the architectures actually implement 8-bit
and 16-bit cmpxchg() and xchg(), it might even be worth trying
to eventually change the interface to not do it at all, but
instead have explicit cmpxchg8() and cmpxchg16() helpers
for the few files that do use them.

     Arnd