linux-kernel - Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Date:   Tue, 6 Apr 2021 17:56:26 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Anup Patel <anup@...infault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Tue, Apr 06, 2021 at 11:50:38AM +0800, Guo Ren wrote:
> On Wed, Mar 31, 2021 at 3:23 PM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Wed, Mar 31, 2021 at 12:35 AM Stafford Horne <shorne@...il.com> wrote:
> > >
> > > I just want to chime in here, there may be a better spot in the thread to
> > > mention this but, for OpenRISC I did implement some generic 8/16-bit xchg code
> > > which I have on my todo list somwhere to replace the other generic
> > > implementations like that in mips.
> > >
> > >   arch/openrisc/include/asm/cmpxchg.h
> > >
> > > The idea would be that architectures just implement these methods:
> > >
> > >   long cmpxchg_u32(*ptr,old,new)
> > >   long xchg_u32(*ptr,val)
> > >
> > > Then the rest of the generic header would implement cmpxchg.
> >
> > I like the idea of generalizing it a little further. I'd suggest staying a
> > little closer to the existing naming here though, as we already have
> > cmpxchg() for the type-agnostic version, and cmpxchg64() for the
> > fixed-length 64-bit version.
> >
> > I think a nice interface between architecture-specific and architecture
> > independent code would be to have architectures provide
> > arch_cmpxchg32()/arch_xchg32() as the most basic version, as well
> > as arch_cmpxchg8()/arch_cmpxchg16()/arch_xchg8()/arch_xchg16()
> > if they have instructions for those.
> >
> > The common code can then build cmpxchg16()/xchg16() on top of
> > either the 16-bit or the 32-bit primitives, and build the cmpxchg()/xchg()
> > wrapper around those (or alternatively we can decide to have them
> > only deal with fixed-32-bit and long/pointer sized atomics).
> I think these emulation codes are suitable for some architectures but not riscv.
> 
> We shouldn't export xchg16/cmpxchg16(emulated by lr.w/sc.w) in riscv,
> We should forbid these sub-word atomic primitive and lets the
> programmers consider their atomic design.

Fair enough, having the generic sub-word emulation would be something that
an architecture can select to use/export.

-Stafford