linux-kernel - Re: [PATCH 3/7] microblaze: Define SMP safe bit operations

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Message-ID: <20200212155309.GA14973@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Feb 2020 16:53:09 +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, Stefan Asserhall <stefan.asserhall@...inx.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 3/7] microblaze: Define SMP safe bit operations

On Wed, Feb 12, 2020 at 04:42:25PM +0100, Michal Simek wrote:
> From: Stefan Asserhall <stefan.asserhall@...inx.com>
> 
> For SMP based system there is a need to have proper bit operations.
> Microblaze is using exclusive load and store instructions.
> 
> Signed-off-by: Stefan Asserhall <stefan.asserhall@...inx.com>
> Signed-off-by: Michal Simek <michal.simek@...inx.com>

> +/*
> + * clear_bit doesn't imply a memory barrier
> + */
> +#define smp_mb__before_clear_bit()	smp_mb()
> +#define smp_mb__after_clear_bit()	smp_mb()

These macros no longer exist.

Also, might I draw your attention to:

  include/asm-generic/bitops/atomic.h

This being a ll/sc arch, I'm thinking that if you do your atomic_t
implementation right, the generic atomic bitop code should be near
optimal.