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

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Message-ID: <CAHB2gtROGuoNzv5f9QrhWX=3ZtZmUM=SAjYhKqP7dTiTTQwkqA@mail.gmail.com>
Date:   Wed, 7 Apr 2021 16:29:12 +0200
From:   Christoph Müllner <christophm30@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, Guo Ren <guoren@...nel.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>,
        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 Wed, Apr 7, 2021 at 11:43 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Tue, Apr 06, 2021 at 09:15:50AM +0200, Peter Zijlstra wrote:
> > Anyway, given you have such a crap architecture (and here I thought
> > RISC-V was supposed to be a modern design *sigh*), you had better go
> > look at the sparc64 atomic implementation which has a software backoff
> > for failed CAS in order to make fwd progress.
>
> It wasn't supposed to be modern.  It was supposed to use boring old
> ideas.  Where it actually did that it is a great ISA, in parts where
> academics actually tried to come up with cool or state of the art
> ideas (interrupt handling, tlb shootdowns, the totally fucked up
> memory model) it turned into a trainwreck.

Gentlemen, please rethink your wording.
RISC-V is neither "crap" nor a "trainwreck", regardless if you like it or not.

The comparison with sparc64 is not applicable, as sparc64 does not
have LL/SC instructions.

Further, it is not the case that RISC-V has no guarantees at all.
It just does not provide a forward progress guarantee for a
synchronization implementation,
that writes in an endless loop to a memory location while trying to
complete an LL/SC
loop on the same memory location at the same time.
If there's a reasonable algorithm, that relies on forward progress in this case,
then we should indeed think about that, but I haven't seen one so far.
The whole MCF lock idea is to actually spin on different memory
locations per CPU
to improve scalability (reduce cacheline bouncing). That's a clear indicator,
that well-scaling synchronization algorithms need to avoid contended cache lines
anyways.

RISC-V defines LR/SC loops consisting of up to 16 instructions as
constrained LR/SC loops.
Such constrained LR/SC loops provide the required forward guarantees,
that are expected
(similar to what other architectures, like AArch64, have).

What RISC-V does not have is sub-word atomics and if required, we
would have to implement
them as LL/SC sequences. And yes, using atomic instructions is
preferred over using LL/SC,
because atomics will tend to perform better (less instructions and
less spilled registers).
But that actually depends on the actual ISA implementation.

Respectfully,
Christoph