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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 28 Nov 2020 09:55:37 -0800
Message-ID: <CALCETrXYkbuJJnDv9ijfT+5tLQ2FOvvN1Ugoh5NwOy+zHp9HXA@mail.gmail.com>
Subject: Re: [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode
To:     Nicholas Piggin <npiggin@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux-MM <linux-mm@kvack.org>, Anton Blanchard <anton@ozlabs.org>
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On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> And get rid of the generic sync_core_before_usermode facility. This is
> functionally a no-op in the core scheduler code, but it also catches
>
> This helper is the wrong way around I think. The idea that membarrier
> state requires a core sync before returning to user is the easy one
> that does not need hiding behind membarrier calls. The gap in core
> synchronization due to x86's sysret/sysexit and lazy tlb mode, is the
> tricky detail that is better put in x86 lazy tlb code.
>
> Consider if an arch did not synchronize core in switch_mm either, then
> membarrier_mm_sync_core_before_usermode would be in the wrong place
> but arch specific mmu context functions would still be the right place.
> There is also a exit_lazy_tlb case that is not covered by this call, which
> could be a bugs (kthread use mm the membarrier process's mm then context
> switch back to the process without switching mm or lazy mm switch).
>
> This makes lazy tlb code a bit more modular.

I have a couple of membarrier fixes that I want to send out today or
tomorrow, and they might eliminate the need for this patch.  Let me
think about this a little bit.  I'll cc you.  The existing code is way
to subtle and the comments are far too confusing for me to be quickly
confident about any of my conclusions :)