[RFC please help] membarrier: Rewrite sync_core_before_usermode()

[Russell King - ARM Linux admin]

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 at efficios.com> wrote:
> >
> > ----- On Dec 27, 2020, at 1:28 PM, Andy Lutomirski luto at kernel.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.

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