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

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Date:   Wed, 7 Apr 2021 18:00:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Müllner <christophm30@...il.com>
Cc:     Christoph Hellwig <hch@...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 07, 2021 at 04:29:12PM +0200, Christoph Müllner wrote:
> 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).

The text quoted by others didn't seem to say such a thing, but whatever.

> 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,

(psudo asm, can't be bothered to figure out the actual syntax)

	# setup r_and_mask, r_or_mask

.L1
	LL r, [word]
	AND r, r, r_and_mask
	OR r, r, r_or_mask
	SC r, [word]
	JNE .L1

is what you need for LL/SC based xchg16, that's less than 16
instructions. If RISC-V guarantees fwd progress on that, good, write it
like that and lets end this thread.

The fact that this is apparently hard, is not good.