linux-kernel - Re: [PATCH 0/7] microblaze: Define SMP safe operations

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Message-ID: <20200212163800.GY14946@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Feb 2020 17:38:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Simek <michal.simek@...inx.com>
Cc:     linux-kernel@...r.kernel.org, monstr@...str.eu, git@...inx.com,
        arnd@...db.de, Allison Randal <allison@...utok.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Shubhrajyoti Datta <shubhrajyoti.datta@...inx.com>,
        Stefan Asserhall <stefan.asserhall@...inx.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 0/7] microblaze: Define SMP safe operations

On Wed, Feb 12, 2020 at 05:08:52PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 12, 2020 at 04:42:22PM +0100, Michal Simek wrote:
> 
> > Microblaze has 32bit exclusive load/store instructions which should be used
> > instead of irq enable/disable. For more information take a look at
> > https://www.xilinx.com/support/documentation/sw_manuals/xilinx2019_2/ug984-vivado-microblaze-ref.pdf
> > starting from page 25.
> 
> >  arch/microblaze/include/asm/Kbuild           |   1 -
> >  arch/microblaze/include/asm/atomic.h         | 265 ++++++++++++++++++-
> >  arch/microblaze/include/asm/bitops.h         | 189 +++++++++++++
> >  arch/microblaze/include/asm/cmpxchg.h        |  87 ++++++
> >  arch/microblaze/include/asm/cpuinfo.h        |   2 +-
> >  arch/microblaze/include/asm/pgtable.h        |  19 +-
> >  arch/microblaze/include/asm/spinlock.h       | 240 +++++++++++++++++
> >  arch/microblaze/include/asm/spinlock_types.h |  25 ++
> >  arch/microblaze/kernel/cpu/cache.c           | 154 ++++++-----
> >  arch/microblaze/kernel/cpu/cpuinfo.c         |  38 ++-
> >  arch/microblaze/kernel/cpu/mb.c              | 207 ++++++++-------
> >  arch/microblaze/kernel/timer.c               |   2 +-
> >  arch/microblaze/mm/consistent.c              |   8 +-
> >  13 files changed, 1040 insertions(+), 197 deletions(-)
> >  create mode 100644 arch/microblaze/include/asm/bitops.h
> >  create mode 100644 arch/microblaze/include/asm/spinlock.h
> >  create mode 100644 arch/microblaze/include/asm/spinlock_types.h
> 
> I'm missing asm/barrier.h
> 
> Also that PDF (thanks for that!), seems light on memory ordering
> details.
> 
> Your comment:
> 
> +/*
> + * clear_bit doesn't imply a memory barrier
> + */
> 
> worries me, because that would imply your ll/sc does not impose order,
> but then you also don't have any explicit barriers in your locking
> primitives or atomics where required.
> 
> In the PDF I only find MBAR; is that what smp_mb() ends up being?

Bah, I'm sure I did patches at some point that made
asm-generic/barrier.h #error if you didn't define at least one memory
barrier on CONFIG_SMP, but it seems that all got lost somewhere.