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

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Date:   Thu, 13 Feb 2020 10:11:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stefan Asserhall <stefana@...inx.com>
Cc:     Michal Simek <michals@...inx.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "monstr@...str.eu" <monstr@...str.eu>, git <git@...inx.com>,
        "arnd@...db.de" <arnd@...db.de>,
        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 Thu, Feb 13, 2020 at 09:01:21AM +0000, Stefan Asserhall wrote:
> The comment in the generic bitops.h says "You should recode these in the
> native assembly language, if at all possible". I don't think using the generic
> implementation will be as efficient as the current arch specific one.

That is a very crusty old recommendation. Please look at the compiler
generated code.

We've extended the atomic_t operations the past few years and Will wrote
the generic atomic bitops for Arm64, we're looking to convert more LL/SC
archs to them.

There is currently one known issue with it, but Will has a patch-set
pending to solve that (IIRC that only matters if you have stack
protector on).

Also see this thread:

  https://lkml.kernel.org/r/875zimp0ay.fsf@mpe.ellerman.id.au

And these patches:

  https://lkml.kernel.org/r/20200123153341.19947-1-will@kernel.org