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

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Message-ID: <YGG5c4QGq6q+lKZI@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Mar 2021 13:26:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     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>,
        Arnd Bergmann <arnd@...db.de>, Anup Patel <anup@...infault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Mon, Mar 29, 2021 at 07:19:29PM +0800, Guo Ren wrote:
> On Mon, Mar 29, 2021 at 3:50 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sat, Mar 27, 2021 at 06:06:38PM +0000, guoren@...nel.org wrote:
> > > From: Guo Ren <guoren@...ux.alibaba.com>
> > >
> > > Some architectures don't have sub-word swap atomic instruction,
> > > they only have the full word's one.
> > >
> > > The sub-word swap only improve the performance when:
> > > NR_CPUS < 16K
> > >  *  0- 7: locked byte
> > >  *     8: pending
> > >  *  9-15: not used
> > >  * 16-17: tail index
> > >  * 18-31: tail cpu (+1)
> > >
> > > The 9-15 bits are wasted to use xchg16 in xchg_tail.
> > >
> > > Please let architecture select xchg16/xchg32 to implement
> > > xchg_tail.
> >
> > So I really don't like this, this pushes complexity into the generic
> > code for something that's really not needed.
> >
> > Lots of RISC already implement sub-word atomics using word ll/sc.
> > Obviously they're not sharing code like they should be :/ See for
> > example arch/mips/kernel/cmpxchg.c.
> I see, we've done two versions of this:
>  - Using cmpxchg codes from MIPS by Michael
>  - Re-write with assembly codes by Guo
> 
> But using the full-word atomic xchg instructions implement xchg16 has
> the semantic risk for atomic operations.

What? -ENOPARSE

> > Also, I really do think doing ticket locks first is a far more sensible
> > step.
> NACK by Anup

Who's he when he's not sending NAKs ?