linux-kernel - Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu

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Date:   Tue, 17 Oct 2023 14:53:33 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     Nadav Amit <namit@...are.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Tue, 17 Oct 2023 at 14:06, Uros Bizjak <ubizjak@...il.com> wrote:
>
> But adding the attached patch on top of both patches boots OK.

Funky.

Mind adding a

        WARN_ON_ONCE(!active_mm);

to there to give a nice backtrace for the odd NULL case.

That code *is* related to 'current', in how we do

        tsk = current;
...
        local_irq_disable();
        active_mm = tsk->active_mm;
        tsk->active_mm = mm;
        tsk->mm = mm;
...
        activate_mm(active_mm, mm);
...
        mmdrop_lazy_tlb(active_mm);

but I don't see how 'active_mm' could *poossibly* be validly NULL
here, and why caching 'current' would matter and change it.

Strange.

Hmm. We do set

        tsk->active_mm = NULL;

in copy_mm(), and then we have that odd kernel thread case:

        /*
         * Are we cloning a kernel thread?
         *
         * We need to steal a active VM for that..
         */
        oldmm = current->mm;
        if (!oldmm)
                return 0;

but none of this should even matter, because by the time we actually
*schedule* that thread, we'll set active_mm to the right thing.

Can anybody see what's up?

             Linus