linux-kernel - Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks

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Message-ID: <20200702080219.GB16113@willie-the-truck>
Date:   Thu, 2 Jul 2020 09:02:19 +0100
From:   Will Deacon <will@...nel.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Anton Blanchard <anton@...abs.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks

On Thu, Jul 02, 2020 at 05:48:36PM +1000, Nicholas Piggin wrote:
> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h
> new file mode 100644
> index 000000000000..f84da77b6bb7
> --- /dev/null
> +++ b/arch/powerpc/include/asm/qspinlock.h
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_POWERPC_QSPINLOCK_H
> +#define _ASM_POWERPC_QSPINLOCK_H
> +
> +#include <asm-generic/qspinlock_types.h>
> +
> +#define _Q_PENDING_LOOPS	(1 << 9) /* not tuned */
> +
> +#define smp_mb__after_spinlock()   smp_mb()
> +
> +static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
> +{
> +	smp_mb();
> +	return atomic_read(&lock->val);
> +}

Why do you need the smp_mb() here?

Will