Re: [RFC] LKMM: Add volatile_if()

Will Deacon
2021-06-04T15:14:04Z

On Fri, Jun 04, 2021 at 03:56:16PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 02:44:22PM +0100, Will Deacon wrote:
> > On Fri, Jun 04, 2021 at 01:31:48PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jun 04, 2021 at 11:44:00AM +0100, Will Deacon wrote:
> > > > On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> > >
> > > > > Usage of volatile_if requires the @cond to be headed by a volatile load
> > > > > (READ_ONCE() / atomic_read() etc..) such that the compiler is forced to
> > > > > emit the load and the branch emitted will have the required
> > > > > data-dependency. Furthermore, volatile_if() is a compiler barrier, which
> > > > > should prohibit the compiler from lifting anything out of the selection
> > > > > statement.
> > > >
> > > > When building with LTO on arm64, we already upgrade READ_ONCE() to an RCpc
> > > > acquire. In this case, it would be really good to avoid having the dummy
> > > > conditional branch somehow, but I can't see a good way to achieve that.
> > >
> > > #ifdef CONFIG_LTO
> > > /* Because __READ_ONCE() is load-acquire */
> > > #define volatile_cond(cond) (cond)
> > > #else
> > > ....
> > > #endif
> > >
> > > Doesn't work? Bit naf, but I'm thinking it ought to do.
> >
> > The problem is with relaxed atomic RMWs; we don't upgrade those to acquire
> > atm as they're written in asm, but we'd need volatile_cond() to work with
> > them. It's a shame, because we only have RCsc RMWs on arm64, so it would
> > be a bit more expensive.
>
> Urgh, I see. Compiler can't really help in that case either I'm afraid.
> They'll never want to modify loads that originate in an asm(). They'll
> say to use the C11 _Atomic crud.

Indeed. That's partly what led me down the route of thinking about "control
ordering" to sit between relaxed and acquire. So you have READ_ONCE_CTRL()
instead of this, but then we can't play your asm goto trick.

If we could push the memory access _and_ the branch down into the new
volatile_if helper, a bit like we do for smp_cond_load_*(), that would
help but it makes the thing a lot harder to use.

In fact, maybe it's actually necessary to bundle the load and branch
together. I looked at some of the examples of compilers breaking control
dependencies from memory-barriers.txt and the "boolean short-circuit"
example seems to defeat volatile_if:

void foo(int *x, int *y)
{
volatile_if (READ_ONCE(*x) || 1 > 0)
WRITE_ONCE(*y, 42);
}

Although we get a conditional branch emitted, it's headed by an immediate
move instruction and the result of the load is discarded:

38: d503233f paciasp
3c: b940001f ldr wzr, [x0]
40: 52800028 mov w8, #0x1 // #1
44: b5000068 cbnz x8, 50 <foo+0x18>
48: d50323bf autiasp
4c: d65f03c0 ret
50: d503249f bti j
54: 52800548 mov w8, #0x2a // #42
58: b9000028 str w8, [x1]
5c: d50323bf autiasp
60: d65f03c0 ret

Will