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Date:   Thu, 21 Oct 2021 17:14:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Waiman Long <longman@redhat.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Guo Ren <guoren@kernel.org>,
        Palmer Dabbelt <palmerdabbelt@google.com>,
        Anup Patel <anup@brainfault.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Christoph =?iso-8859-1?Q?M=FCllner?= <christophm30@gmail.com>,
        Stafford Horne <shorne@gmail.com>
Subject: Re: [PATCH] locking: Generic ticket lock
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On Thu, Oct 21, 2021 at 03:49:51PM +0200, Arnd Bergmann wrote:
> On Thu, Oct 21, 2021 at 3:05 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Therefore provide ticket locks, which depend on a single atomic
> > operation (fetch_add) while still providing fairness.
> 
> Nice!
> 
> Aside from the qspinlock vs ticket-lock question, can you describe the
> tradeoffs between this generic ticket lock and a custom implementation
> in architecture code? Should we convert most architectures over
> to the generic code in the long run, or is there something they
> can usually do better with an inline asm based ticket lock

I think for a load-store arch this thing should generate pretty close to
optimal code. x86 can do ticket_unlock() slightly better using a single
INCW (or ADDW 1) on the owner subword, where this implementation will to
separate load-add-store instructions.

If that is actually measurable is something else entirely.

> or a trivial test-and-set?

If your SMP arch is halfway sane (no fwd progress issues etc..) then
ticket should behave well and avoid the starvation/variablilty of TaS
lock.

The big exception there is virtualized architectures, ticket is
absolutely horrendous for 'guests' (any fair lock is for that matter).