linux-kernel - Re: [RFC please help] membarrier: Rewrite sync_core_before

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Date:   Mon, 28 Dec 2020 10:25:37 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [RFC please help] membarrier: Rewrite sync_core_before_usermode()

On Sun, Dec 27, 2020 at 01:36:13PM -0800, Andy Lutomirski wrote:
> On Sun, Dec 27, 2020 at 12:18 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > ----- On Dec 27, 2020, at 1:28 PM, Andy Lutomirski luto@...nel.org wrote:
> >
> 
> > >
> > > I admit that I'm rather surprised that the code worked at all on arm64,
> > > and I'm suspicious that it has never been very well tested.  My apologies
> > > for not reviewing this more carefully in the first place.
> >
> > Please refer to Documentation/features/sched/membarrier-sync-core/arch-support.txt
> >
> > It clearly states that only arm, arm64, powerpc and x86 support the membarrier
> > sync core feature as of now:
> 
> Sigh, I missed arm (32).  Russell or ARM folks, what's the right
> incantation to make the CPU notice instruction changes initiated by
> other cores on 32-bit ARM?

You need to call flush_icache_range(), since the changes need to be
flushed from the data cache to the point of unification (of the Harvard
I and D), and the instruction cache needs to be invalidated so it can
then see those updated instructions. This will also take care of the
necessary barriers that the CPU requires for you.

... as documented in Documentation/core-api/cachetlb.rst and so
should be available on every kernel supported CPU.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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