linux-kernel - Re: [PATCH 0/5] Generic Ticket Spinlocks

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Date:   Thu, 17 Mar 2022 10:16:50 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Palmer Dabbelt <palmer@...osinc.com>
Cc:     linux-riscv <linux-riscv@...ts.infradead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Arnd Bergmann <arnd@...db.de>,
        Jisheng Zhang <jszhang@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Openrisc <openrisc@...ts.librecores.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 0/5] Generic Ticket Spinlocks

On Thu, Mar 17, 2022 at 12:25 AM Palmer Dabbelt <palmer@...osinc.com> wrote:
>
> Peter sent an RFC out about a year ago
> <https://lore.kernel.org/lkml/YHbBBuVFNnI4kjj3@hirez.programming.kicks-ass.net/>,
> but after a spirited discussion it looks like we lost track of things.
> IIRC there was broad consensus on this being the way to go, but there
> was a lot of discussion so I wasn't sure.  Given that it's been a year,
> I figured it'd be best to just send this out again formatted a bit more
> explicitly as a patch.
>
> This has had almost no testing (just a build test on RISC-V defconfig),
> but I wanted to send it out largely as-is because I didn't have a SOB
> from Peter on the code.  I had sent around something sort of similar in
> spirit, but this looks completely re-written.  Just to play it safe I
> wanted to send out almost exactly as it was posted.  I'd probably rename
> this tspinlock and tspinlock_types, as the mis-match kind of makes my
> eyes go funny, but I don't really care that much.  I'll also go through
> the other ports and see if there's any more candidates, I seem to
> remember there having been more than just OpenRISC but it's been a
> while.
>
> I'm in no big rush for this and given the complex HW dependencies I
> think it's best to target it for 5.19, that'd give us a full merge
> window for folks to test/benchmark it on their systems to make sure it's
> OK.  RISC-V has a forward progress guarantee so we should be safe, but
> these can always trip things up.

This all looks good to me, feel free to merge the asm-generic
bits through the riscv tree.

Regarding the naming, my preference would be to just use
this version in place of the (currently useless) asm-generic/spinlock.h,
and just naming it arch_spin_lock() etc.

This way, converting an architecture to the generic ticket lock can
be done simply by removing its custom asm/spinlock.h. Or it
could stay with the current name, but then have a new
asm-generic/spinlock.h that just includes both asm/ticket_lock.h
and asm/qrwlock.h.

      Arnd