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

[Paul E. McKenney]

On Mon, Jul 06, 2020 at 06:05:57PM +0100, Dave Martin wrote:
> On Mon, Jul 06, 2020 at 09:34:55AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 06, 2020 at 05:00:23PM +0100, Dave Martin wrote:
> > > On Thu, Jul 02, 2020 at 08:23:02AM +0100, Will Deacon wrote:
> > > > On Wed, Jul 01, 2020 at 06:07:25PM +0100, Dave P Martin wrote:
> > > > > On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon 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
> > > > > > 
> > > > > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> > > > > > new file mode 100644
> > > > > > index 000000000000..515e360b01a1
> > > > > > --- /dev/null
> > > > > > +++ b/arch/arm64/include/asm/rwonce.h
> > > > > > @@ -0,0 +1,63 @@
> > > > > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > > > > +/*
> > > > > > + * Copyright (C) 2020 Google LLC.
> > > > > > + */
> > > > > > +#ifndef __ASM_RWONCE_H
> > > > > > +#define __ASM_RWONCE_H
> > > > > > +
> > > > > > +#ifdef CONFIG_CLANG_LTO
> > > > > 
> > > > > Don't we have a generic option for LTO that's not specific to Clang.
> > > > 
> > > > /me looks at the LTO series some more
> > > > 
> > > > Oh yeah, there's CONFIG_LTO which is selected by CONFIG_LTO_CLANG, which is
> > > > the non-typoed version of the above. I can switch this to CONFIG_LTO.
> > > > 
> > > > > Also, can you illustrate code that can only be unsafe with Clang LTO?
> > > > 
> > > > I don't have a concrete example, but it's an ongoing concern over on the LTO
> > > > thread [1], so I cooked this to show one way we could deal with it. The main
> > > > concern is that the whole-program optimisations enabled by LTO may allow the
> > > > compiler to enumerate possible values for a pointer at link time and replace
> > > > an address dependency between two loads with a control dependency instead,
> > > > defeating the dependency ordering within the CPU.
> > > 
> > > Why can't that happen without LTO?
> > 
> > Because without LTO, the compiler cannot see all the pointers all at
> > the same time due to their being in different translation units.
> > 
> > But yes, if the compiler could see all the pointer values and further
> > -know- that it was seeing all the pointer values, these optimizations
> > could happen even without LTO.  But it is quite easy to make sure that
> > the compiler thinks that there are additional pointer values that it
> > does not know about.
> 
> Yes of course, but even without LTO the compiler can still apply this
> optimisation to everything visible in the translation unit, and that can
> drift as people refactor code over time.
> 
> Convincing the compiler there are other possible values doesn't help.
> Even in
> 
> int foo(int *p)
> {
> 	asm ("" : "+r" (p));
> 	return *p;
> }
> 
> Can't the compiler still generate something like this:
> 
> 	switch (p) {
> 	case &foo:
> 		return foo;
> 
> 	case &bar:
> 		return bar;
> 
> 	default:
> 		return *p;
> 	}
> 
> ...in which case we still have the same lost ordering guarantee that
> we were trying to enforce.
> 
> If foo and bar already happen to be in registers and profiling shows
> that &foo and &bar are the most likely value of p then this might be
> a reasonable optimisation in some situations, irrespective of LTO.

Agreed, the additional information from profile-driven optimization
can be just as damaging as that from LTO.

> The underlying problem here seems to be that the necessary ordering
> rule is not part of what passes for the C memory model prior to C11.
> If we want to control the data flow, don't we have to wrap the entire
> dereference in a macro?

Yes, exactly.  Because we are relying on things that are not guaranteed
by the C memory model, we need to pay attention to the implementations.
As I have said elsewhere, the price of control dependencies is eternal
vigilance.

And this also applies, to a lesser extent, to address and data
dependencies, which are also not well supported by the C standard.

There is one important case in which the C memory model -does- support
control dependencies, and that is when the dependent write is a normal
C-language write that is not involved in a data race.  In that case,
if the compiler broke the control dependency, it might have introduced
a data race, which it is forbidden to do.  However, this rule can also
be broken when the compiler knows too much, as it might be able to prove
that breaking the dependency won't introduce a data race.  In that case,
according to the standard, it is free to break the dependency.

> > > > We likely won't realise if/when this goes wrong, other than impossible to
> > > > debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be
> > > > able to detect this sort of thing happening at build time, and perhaps
> > > > even prevent it with compiler options or annotations, but none of that is
> > > > close to being available and I'm keen to progress the LTO patches in the
> > > > meantime because they are a requirement for CFI.
> > > 
> > > My concern was not so much why LTO makes things dangerous, as why !LTO
> > > makes things safe...
> > 
> > Because ignorant compilers are safe compilers!  ;-)
> 
> AFAICT ignorance is no gurantee of ordering in general -- the compiler
> is free to speculatively invent knowledge any place that the language
> spec allows it to.  !LTO doesn't stop this happening.

Agreed, according to the standard, the compiler has great freedom.

We have two choices: (1) Restrict ourselves to live within the confines of
the standard or (2) Pay continued close attention to the implementation.
We have made different choices at different times, but for many ordering
situations we have gone with door #2.

Me, I have been working to get the standard to better support our
use case.  This is at best slow going.  But don't take my word for it,
ask Will.

> Hopefully some of the knowledge I invented in my reply is valid...

It is.  It is just that there are multiple valid strategies, and the
Linux kernel is currently taking a mixed-strategy approach.

							Thanx, Paul