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From:   Nadav Amit <namit@vmware.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
CC:     Uros Bizjak <ubizjak@gmail.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
Thread-Topic: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
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Date:   Tue, 17 Oct 2023 22:06:51 +0000
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> On Oct 18, 2023, at 12:53 AM, Linus Torvalds <torvalds@linux-foundation.o=
rg> wrote:
>=20
>=20
> 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.
>=20
> Can anybody see what's up?

Could it be related to exec_mmap() -> exec_mm_release() -> mm_release() -> =
deactivate_mm() ?

#define deactivate_mm(tsk, mm)                  \
do {                                            \
        if (!tsk->vfork_done)                   \
                shstk_free(tsk);                \
        load_gs_index(0);                       \
        loadsegment(fs, 0);                     \
} while (0)

We change gs_index(), so perhaps it affects later GS reads. There is also t=
his
X86_BUG_NULL_SEG. Need to dive deeper; just initial thoughts though (i.e., =
I might be
completely off).