linux-kernel - Re: [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket & queued)

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Message-ID: <CAJF2gTQLqN2QnHYyPMx91vraR3cOhf2DOnnC3=5C18ACU32Qaw@mail.gmail.com>
Date:   Fri, 1 Jul 2022 20:18:26 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Waiman Long <longman@...hat.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket
 & queued)

On Wed, Jun 29, 2022 at 4:30 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Wed, Jun 29, 2022 at 10:24 AM Guo Ren <guoren@...nel.org> wrote:
> > On Wed, Jun 29, 2022 at 3:09 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > On Wed, Jun 29, 2022 at 3:34 AM Waiman Long <longman@...hat.com> wrote:
> > >
> > > From looking at the header file dependencies on arm64, I know that
> > > putting jump labels into core infrastructure like the arch_spin_lock()
> > > makes a big mess of indirect includes and measurably slows down
> > > the kernel build.
> > arm64 needn't combo spinlock, it could use pure qspinlock with keeping
> > current header files included.
>
> arm64 has a different problem: there are two separate sets of atomic
> instructions, and the decision between those is similarly done using
> jump labels. I definitely like the ability to choose between qspinlock
> and ticket spinlock on arm64 as well. This can be done as a
> compile-time choice, but both of them still depend on jump labels.
1. xchg use ALTERNATIVE, but cmpxchg to jump labels.
2. arm64 is still using qspinlock when ll/sc, and I think they give
strong enough fwd guarantee with "prfm pstl1strm".
But another question is if ll/sc could give enough strong fwd
guarantee, why arm64 introduce LSE, for code size reduction? Why
instructions fusion technology is not enough?


>
>         Arnd



--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/