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

[Paul E. McKenney]

On Tue, Jul 07, 2020 at 11:29:15AM +0100, Dave Martin wrote:
> On Mon, Jul 06, 2020 at 10:36:28AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 06, 2020 at 06:05:57PM +0100, Dave Martin wrote:

[ . . . ]

> > > 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.
> 
> Which only matters because the C abstract machine may not match reality.
> 
> LTO has no bearing on the abstract machine though.
> 
> If specific compiler options etc. can be added to inhibit the
> problematic optimisations, that would be ideal.  I guess that can't
> happen overnight though.

Sadly, I must agree.

> > > > > > 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.
> 
> I can believe it.  They want to enable optimisations rather than prevent
> them...

Right in one!  ;-)

> > > 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.
> 
> Ack.  The hope that there is a correct way to fix everything dies
> hard ;)

Either that, or one slowly degrades ones definition of "correct".  :-/

> Life was cosier before I started trying to reason about language specs.

Same here!

							Thanx, Paul