linux-kernel - Re: [PATCH 2/3] ARM: Move thread_info into task

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Message-ID: <874kawcssr.fsf@keithp.com>
Date:   Tue, 07 Sep 2021 08:24:36 -0700
From:   Keith Packard <keithp@...thp.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Abbott Liu <liuwenliang@...wei.com>,
        Alexander Sverdlin <alexander.sverdlin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Hartley Sweeten <hsweeten@...ionengravers.com>,
        Jens Axboe <axboe@...nel.dk>, Jian Cai <jiancai@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Rob Herring <robh@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Wolfram Sang (Renesas)" <wsa+renesas@...g-engineering.com>,
        YiFei Zhu <yifeifz2@...inois.edu>
Subject: Re: [PATCH 2/3] ARM: Move thread_info into task_struct (v7 only)

Ard Biesheuvel <ardb@...nel.org> writes:

> Sure, so it is precisely for that reason that it is better to isolate
> changes that can be isolated.

I'll go ahead and split this apart then; that is how I did development,
after all.

> All the time. 'current' essentially never changes value from the POV
> of code running in task context, so there is usually no reason to care
> about preemption/migration when referring to it. Using per-CPU
> variables is what creates the problem here.

Thanks for helping me -- I just got the wrong model stuck in my head
over the weekend somehow.

If I do have this figured out, we should be able to stick the
per_cpu_offset value in thread_info and use TPIDRPRW to hold 'current'
as code using per_cpu_offset should already be disabling
preemption. That should be an easier change than putting a kernel
pointer in a user-visible register.

> Given that we are already relying on the MP extensions for this
> anyway, I personally think that using another thread ID register to
> carry 'current' is a reasonable approach as well, since it would also
> allow us to get per-task stack protector support into the compiler.
> But I would like to hear from some other folks on cc as well.

That would be awesome; I assume that doesn't require leaving
per_cpu_offset in a thread ID register?

In any case, I'll give my plan a try, and then see about trying your
plan as well so I can compare the complexity of the two solutions.

-- 
-keith

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