Re: [RFC] LKMM: Add volatile_if()

From: Paul E. McKenney
Date: Mon Jun 07 2021 - 16:16:36 EST


On Mon, Jun 07, 2021 at 03:51:44PM -0400, Alan Stern wrote:
> On Mon, Jun 07, 2021 at 01:23:35PM -0500, Segher Boessenkool wrote:
> > On Mon, Jun 07, 2021 at 08:27:12AM -0700, Paul E. McKenney wrote:
> > > > > > > The barrier() thing can work - all we need to do is to simply make it
> > > > > > > impossible for gcc to validly create anything but a conditional
> > > > > > > branch.
> >
> > > > > What would you suggest as a way of instructing the compiler to emit the
> > > > > conditional branch that we are looking for?
> > > >
> > > > You write it in the assembler code.
> > > >
> > > > Yes, it sucks. But it is the only way to get a branch if you really
> > > > want one. Now, you do not really need one here anyway, so there may be
> > > > some other way to satisfy the actual requirements.
> > >
> > > Hmmm... What do you see Peter asking for that is different than what
> > > I am asking for? ;-)
> >
> > I don't know what you are referring to, sorry?
> >
> > I know what you asked for: literally some way to tell the compiler to
> > emit a conditional branch. If that is what you want, the only way to
> > make sure that is what you get is by writing exactly that in assembler.
>
> That's not necessarily it.
>
> People would be happy to have an easy way of telling the compiler that
> all writes in the "if" branch of an if statement must be ordered after
> any reads that the condition depends on. Or maybe all writes in either
> the "if" branch or the "else" branch. And maybe not all reads that the
> condition depends on, but just the reads appearing syntactically in the
> condition. Or maybe even just the volatile reads appearing in the
> condition. Nobody has said exactly.
>
> The exact method used for doing this doesn't matter. It could be
> accomplished by treating those reads as load-acquires. Or it could be
> done by ensuring that the object code contains a dependency (control or
> data) from the reads to the writes. Or it could be done by treating
> the writes as store-releases. But we do want the execution-time
> penalty to be small.
>
> In short, we want to guarantee somehow that the conditional writes are
> not re-ordered before the reads in the condition. (But note that
> "conditional writes" includes identical writes occurring in both
> branches.)

What Alan said! ;-)

Thanx, Paul