Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-07T18:08:31Z

On Mon, Jun 07, 2021 at 05:02:53PM +0100, Will Deacon wrote:
> Hi Paul,
>
> On Mon, Jun 07, 2021 at 08:25:33AM -0700, Paul E. McKenney wrote:
> > On Mon, Jun 07, 2021 at 12:52:35PM +0100, Will Deacon wrote:
> > > It's the conditional instructions that are more fun. For example, the CSEL
> > > instruction:
> > >
> > > CSEL X0, X1, X2, <cond>
> > >
> > > basically says:
> > >
> > > if (cond)
> > > X0 = X1;
> > > else
> > > X0 = X2;
> > >
> > > these are just register-register operations, but the idea is that the CPU
> > > can predict that "branching event" inside the CSEL instruction and
> > > speculatively rename X0 while waiting for the condition to resolve.
> > >
> > > So then you can add loads and stores to the mix along the lines of:
> > >
> > > LDR X0, [X1] // X0 = *X1
> > > CMP X0, X2
> > > CSEL X3, X4, X5, EQ // X3 = (X0 == X2) ? X4 : X5
> > > STR X3, [X6] // MUST BE ORDERED AFTER THE LOAD
> > > STR X7, [X8] // Can be reordered
> > >
> > > (assuming X1, X6, X8 all point to different locations in memory)
> > >
> > > So now we have a dependency from the load to the first store, but the
> > > interesting part is that the last store is _not_ ordered wrt either of the
> > > other two memory accesses, whereas it would be if we used a conditional
> > > branch instead of the CSEL. Make sense?
> >
> > And if I remember correctly, this is why LKMM orders loads in the
> > "if" condition only with stores in the "then" and "else" clauses,
> > not with stores after the end of the "if" statement. Or is there
> > some case that I am missing?
>
> It's not clear to me that such a restriction prevents the compiler from
> using any of the arm64 conditional instructions in place of the conditional
> branch in such a way that you end up with an "independent" store in the
> assembly output constructed from two stores on the "then" and "else" paths
> which the compiler determined where the same.
>
> > > Now, obviously the compiler is blissfully unaware that conditional
> > > data processing instructions can give rise to dependencies than
> > > conditional branches, so the question really is how much do we need to
> > > care in the kernel?
> > >
> > > My preference is to use load-acquire instead of control dependencies so
> > > that we don't have to worry about this, or any future relaxations to the
> > > CPU architecture, at all.
> >
> > From what I can see, ARMv8 has DMB(LD) and DMB(ST). Does it have
> > something like a DMB(LD,ST) that would act something like powerpc lwsync?
> >
> > Or are you proposing rewriting the "if" conditions to upgrade
> > READ_ONCE() to smp_load_acquire()? Or something else?
> >
> > Just trying to find out exactly what you are proposing. ;-)
>
> Some options are:
>
> (1) Do nothing until something actually goes wrong (and hope we spot/debug it)
>
> (2) Have volatile_if force a conditional branch, assuming that it solves
> the problem and doesn't hurt codegen (I still haven't convinced myself
> for either case)
>
> (3) Upgrade READ_ONCE() to RCpc acquire, relaxed atomic RMWs to RCsc
> acquire on arm64
>
> (4) Introduce e.g. READ_ONCE_CTRL(), atomic_add_return_ctrl() etc
> specifically for control dependencies and upgrade only those for
> arm64
>
> (5) Work to get toolchain support for dependency ordering and use that
>
> I'm suggesting (3) or (4) because, honestly, it feels like we're being
> squeezed from both sides with both the compiler and the hardware prepared
> to break control dependencies.

I will toss out this as well:

(6) Create a volatile_if() that does not support an "else" clause,
thus covering all current use cases and avoiding some of the
same-store issues. Which in the end might or might not help,
but perhaps worth looking into.

Thanx, Paul