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From:   Heiko =?ISO-8859-1?Q?St=FCbner?= <heiko@sntech.de>
To:     Arnd Bergmann <arnd@arndb.de>, linux-riscv@lists.infradead.org
Cc:     guoren@kernel.org, peterz@infradead.org, mingo@redhat.com,
        Will Deacon <will@kernel.org>, longman@redhat.com,
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        stefan.kristiansson@saunalahti.fi, shorne@gmail.com,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>, aou@eecs.berkeley.edu,
        Arnd Bergmann <arnd@arndb.de>,
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        linux-arch@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org, Palmer Dabbelt <palmer@rivosinc.com>,
        Palmer Dabbelt <palmer@rivosinc.com>
Subject: Re: [PATCH v4 2/7] asm-generic: qspinlock: Indicate the use of mixed-size atomics
Date:   Wed, 04 May 2022 14:02:00 +0200
Message-ID: <7375410.EvYhyI6sBW@diego>
In-Reply-To: <20220430153626.30660-3-palmer@rivosinc.com>
References: <20220430153626.30660-1-palmer@rivosinc.com> <20220430153626.30660-3-palmer@rivosinc.com>
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Am Samstag, 30. April 2022, 17:36:21 CEST schrieb Palmer Dabbelt:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> The qspinlock implementation depends on having well behaved mixed-size
> atomics.  This is true on the more widely-used platforms, but these
> requirements are somewhat subtle and may not be satisfied by all the
> platforms that qspinlock is used on.
> 
> Document these requirements, so ports that use qspinlock can more easily
> determine if they meet these requirements.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> ---
>  include/asm-generic/qspinlock.h | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index d74b13825501..95be3f3c28b5 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -2,6 +2,37 @@
>  /*
>   * Queued spinlock
>   *
> + * A 'generic' spinlock implementation that is based on MCS locks. An

_For_ an architecture that's ... ?

> + * architecture that's looking for a 'generic' spinlock, please first consider
> + * ticket-lock.h and only come looking here when you've considered all the
> + * constraints below and can show your hardware does actually perform better
> + * with qspinlock.
> + *
> + *

double empty line is probably not necessary

> + * It relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no weaker
> + * than RCtso if you're power), where regular code only expects atomic_t to be
> + * RCpc.
> + *
> + * It relies on a far greater (compared to asm-generic/spinlock.h) set of
> + * atomic operations to behave well together, please audit them carefully to
> + * ensure they all have forward progress. Many atomic operations may default to
> + * cmpxchg() loops which will not have good forward progress properties on
> + * LL/SC architectures.
> + *
> + * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
> + * do. Carefully read the patches that introduced
> + * queued_fetch_set_pending_acquire().
> + *
> + * It also heavily relies on mixed size atomic operations, in specific it
> + * requires architectures to have xchg16; something which many LL/SC
> + * architectures need to implement as a 32bit and+or in order to satisfy the
> + * forward progress guarantees mentioned above.
> + *
> + * Further reading on mixed size atomics that might be relevant:
> + *
> + *   http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
> + *
> + *
>   * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
>   * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
>   *
>