Date:   Tue, 14 Dec 2021 15:40:57 +0000
From:   Will Deacon <will@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org,
        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
Message-ID: <20211214154057.GB15416@willie-the-truck>
References: <YXFli3mzMishRpEq@hirez.programming.kicks-ass.net>
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On Thu, Oct 21, 2021 at 03:05:15PM +0200, Peter Zijlstra wrote:
> 
> There's currently a number of architectures that want/have graduated
> from test-and-set locks and are looking at qspinlock.
> 
> *HOWEVER* qspinlock is very complicated and requires a lot of an
> architecture to actually work correctly. Specifically it requires
> forward progress between a fair number of atomic primitives, including
> an xchg16 operation, which I've seen a fair number of fundamentally
> broken implementations of in the tree (specifically for qspinlock no
> less).
> 
> The benefit of qspinlock over ticket lock is also non-obvious, esp.
> at low contention (the vast majority of cases in the kernel), and it
> takes a fairly large number of CPUs (typically also NUMA) to make
> qspinlock beat ticket locks.
> 
> Esp. things like ARM64's WFE can move the balance a lot in favour of
> simpler locks by reducing the cacheline pressure due to waiters (see
> their smp_cond_load_acquire() implementation for details).
> 
> Unless you've audited qspinlock for your architecture and found it
> sound *and* can show actual benefit, simpler is better.
> 
> Therefore provide ticket locks, which depend on a single atomic
> operation (fetch_add) while still providing fairness.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/asm-generic/qspinlock.h         |   30 +++++++++
>  include/asm-generic/ticket_lock_types.h |   11 +++
>  include/asm-generic/ticket_lock.h       |   97 ++++++++++++++++++++++++++++++++
>  3 files changed, 138 insertions(+)

Huh. I looked quite closely at this a while back but seems like I forgot to
actually reply here. So, given that it doesn't seem to be in linux-next yet:

Acked-by: Will Deacon <will@kernel.org>

Will