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

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Date:   Mon, 29 Mar 2021 20:52:57 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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>,
        Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Mon, Mar 29, 2021 at 7:31 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Mar 29, 2021 at 01:16:53PM +0200, Peter Zijlstra wrote:
> > Anyway, an additional 'funny' is that I suspect you cannot prove fwd
> > progress of the entire primitive with any of this on. But who cares
> > about details anyway.. :/
>
> What's the architectural guarantee on LL/SC progress for RISC-V ?

funct5    | aq | rl   | rs2 |  rs1  | funct3 | rd | opcode
     5          1    1      5       5         3        5          7
LR.W/D  ordering  0     addr    width   dest    AMO
SC.W/D  ordering  src  addr    width   dest    AMO

LR.W loads a word from the address in rs1, places the sign-extended
value in rd, and registers a reservation set—a set of bytes that
subsumes the bytes in the addressed word. SC.W conditionally writes a
word in rs2 to the address in rs1: the SC.W succeeds only if the
reservation is still valid and the reservation set contains the bytes
being written. If the SC.W succeeds, the instruction writes the word
in rs2 to memory, and it writes zero to rd. If the SC.W fails, the
instruction does not write to memory, and it writes a nonzero value to
rd. Regardless of success or failure, executing an SC.W instruction
*invalidates any reservation held by this hart*.

More details, ref:
https://github.com/riscv/riscv-isa-manual

> And what if you double loop it like cmpxchg() ?
Can you give a code snippet?

--
Best Regards
 Guo Ren

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