Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

[Will Deacon]

On Tue, Jun 30, 2020 at 09:47:30PM +0200, Marco Elver wrote:
> On Tue, 30 Jun 2020 at 19:39, Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > When building with LTO, there is an increased risk of the compiler
> > converting an address dependency headed by a READ_ONCE() invocation
> > into a control dependency and consequently allowing for harmful
> > reordering by the CPU.
> >
> > Ensure that such transformations are harmless by overriding the generic
> > READ_ONCE() definition with one that provides acquire semantics when
> > building with LTO.
> >
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/rwonce.h   | 63 +++++++++++++++++++++++++++++++
> >  arch/arm64/kernel/vdso/Makefile   |  2 +-
> >  arch/arm64/kernel/vdso32/Makefile |  2 +-
> >  3 files changed, 65 insertions(+), 2 deletions(-)
> >  create mode 100644 arch/arm64/include/asm/rwonce.h
> 
> This seems reasonable, given we can't realistically tell the compiler
> about dependent loads. What (if any), is the performance impact? I
> guess this also heavily depends on the actual silicon.

Right, it depends both on the CPU micro-architecture and also the workload.
When we ran some basic tests, the overhead wasn't greater than the benefit
seen by enabling LTO, so it seems like a reasonable trade-off (given that
LTO is a dependency for CFI, so it's not just about performance).

Will