Re: [RFC] LKMM: Add volatile_if()

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Jun 07, 2021 at 05:40:37PM -0500, Segher Boessenkool wrote:
> On Mon, Jun 07, 2021 at 01:16:33PM -0700, Paul E. McKenney wrote:
> > 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!  ;-)
> 
> Okay, I'll think about that.
> 
> But you wrote:
> 
> > > > > > > What would you suggest as a way of instructing the compiler to emit the
> > > > > > > conditional branch that we are looking for?
> 
> ... and that is what I answered.  I am sorry if you do not like being
> taken literally, but that is how I read technical remarks: as literally
> what they say.  If you say you want a branch, I take it you want a
> branch!  :-)

When it is the cheapest means of providing the needed ordering, I really
do want a branch.  ;-)

And a branch would implement Alan's "control dependency" above.

							Thanx, Paul