linux-kernel - Re: [PATCH 1/5] asm-generic: qspinlock: Indicate the use of mixed-size atomics

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Message-ID: <67ba2190-dd72-4ad0-32c2-de43418b73a2@redhat.com>
Date:   Thu, 17 Mar 2022 13:46:46 -0400
From:   Waiman Long <longman@...hat.com>
To:     Palmer Dabbelt <palmer@...osinc.com>,
        linux-riscv@...ts.infradead.org, peterz@...radead.org
Cc:     jonas@...thpole.se, stefan.kristiansson@...nalahti.fi,
        shorne@...il.com, mingo@...hat.com, Will Deacon <will@...nel.org>,
        boqun.feng@...il.com, Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, aou@...s.berkeley.edu,
        Arnd Bergmann <arnd@...db.de>, jszhang@...nel.org,
        wangkefeng.wang@...wei.com, openrisc@...ts.librecores.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 1/5] asm-generic: qspinlock: Indicate the use of
 mixed-size atomics

On 3/16/22 19:25, Palmer Dabbelt wrote:
> From: Peter Zijlstra <peterz@...radead.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: Palmer Dabbelt <palmer@...osinc.com>
>
> ---
>
> I have specifically not included Peter's SOB on this, as he sent his
> original patch
> <https://lore.kernel.org/lkml/YHbBBuVFNnI4kjj3@hirez.programming.kicks-ass.net/>
> without one.
> ---
>   include/asm-generic/qspinlock.h | 30 ++++++++++++++++++++++++++++++
>   1 file changed, 30 insertions(+)
>
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index d74b13825501..a7a1296b0b4d 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -2,6 +2,36 @@
>   /*
>    * Queued spinlock
>    *
> + * A 'generic' spinlock implementation that is based on MCS locks. An
> + * 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.
> + *
> + *
> + * 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 ticket-lock.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
>    *
Acked-by: Waiman Long <longman@...hat.com>

Note that it references ticket-lock.h. Perhaps we should reverse the 
order of patches 1 & 2.

Cheers,
Longman