Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-04T14:13:15Z

On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> > +/**
> > + * volatile_if() - Provide a control-dependency
> > + *
> > + * volatile_if(READ_ONCE(A))
> > + * WRITE_ONCE(B, 1);
> > + *
> > + * will ensure that the STORE to B happens after the LOAD of A. Normally a
> > + * control dependency relies on a conditional branch having a data dependency
> > + * on the LOAD and an architecture's inability to speculate STOREs. IOW, this
> > + * provides a LOAD->STORE order.
> > + *
> > + * Due to optimizing compilers extra care is needed; as per the example above
> > + * the LOAD must be 'volatile' qualified in order to ensure the compiler
> > + * actually emits the load, such that the data-dependency to the conditional
> > + * branch can be formed.
> > + *
> > + * Secondly, the compiler must be prohibited from lifting anything out of the
> > + * selection statement, as this would obviously also break the ordering.
> > + *
> > + * Thirdly, and this is the tricky bit, architectures that allow the
> > + * LOAD->STORE reorder must ensure the compiler actually emits the conditional
> > + * branch instruction, this isn't possible in generic.
> > + *
> > + * See the volatile_cond() wrapper.
> > + */
> > +#define volatile_if(cond) if (volatile_cond(cond))
>
> On naming (sorry Paul for forgetting that in the initial mail); while I
> think using the volatile qualifier for the language feature (can we haz
> plz, kthxbai) makes perfect sense, Paul felt that we might use a
> 'better' name for the kernel use, ctrl_dep_if() was proposed.
>
> Let us pain bike sheds :-)

I have felt that pain many times... ;-)

Here is what I see thus far from these two threads:

1. volatile_if() as above. Nice ease of use, but might be suboptimal
on architectures where a branch is slower than an acquire load.

2. #1, but with my preferred name of ctrl_dep_if() instead of
volatile_if().

3. READ_ONCE_CTRL() like back in the old days. This has the
advantage of giving the compiler more information, but has
problems with relaxed atomic RMW operations.

4. A full (fool?) solution based on #3 would also include _ctrl
suffixed atomic RMW operations.

5. Your bikeshed color here!

Thanx, Paul