On Wed, May 30, 2018 at 03:08:43PM -0400, Alan Stern wrote:
> On Wed, 30 May 2018, Paul E. McKenney wrote:
>
> > On Wed, May 30, 2018 at 09:59:28AM -0500, Linus Torvalds wrote:
> > > On Wed, May 30, 2018 at 9:29 AM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > wrote:
> > >
> > > > >
> > > > > Can't we simplify the whole sequence as basically
> > > > >
> > > > > A
> > > > > if (!B)
> > > > > D
> > > > >
> > > > > for that "not B" case, and just think about that. IOW, let's ignore the
> > > > > whole "not executed" code.
> > >
> > > > Your listing is slightly misleading.
> > >
> > > No. You're confused.
> > >
> > > You're confused because you're conflating two *entirely* different things.
> > >
> > > You're conflating the static source code with the dynamic execution. They
> > > are NOT THE SAME.
> >
> > I am going to go out on a limb and assert that Linus is talking about
> > what gcc and hardware do, while Alan is talking about what the tool and
> > memory model do.
>
> Indeed. The very first line Linus quoted in his first reply to me
> (elided above) was:
>
> Putting this into herd would be extremely difficult, if not impossible,
> because it involves analyzing code that was not executed.
>
> It should be clear from this that I was talking about herd. Not gcc or
> real hardware.
>
> (After rereading his message a few times, I'm not sure exactly what
> Linus was talking about...)
>From what I could see, real compilers and real hardware. ;-)
> > In a perfect world, these would be the same, but it
> > appears that the world might not be perfect just now.
> >
> > My current guess is that we need to change the memory-model tool. If
> > that is the case, here are some possible resolutions:
> >
> > 1. Make herd's C-language control dependencies work the same as its
> > assembly language, so that they extend beyond the end of the
> > "if" statement. I believe that this would make Roman's case
> > work, but it could claim that other situations are safe that
> > are actually problematic due to compiler optimizations.
> >
> > The fact that the model currently handles only READ_ONCE()
> > and WRITE_ONCE() and not unmarked reads and writes make this
> > option more attractive than it otherwise be, compilers not
> > being allowed to reorder volatile accesses, but we are likely
> > to introduce unmarked accesses sometime in the future.
>
> Preserving the order of volatile accesses isn't sufficient. The
> compiler is still allowed to translate
>
> r1 = READ_ONCE(x);
> if (r1) {
> ...
> }
> WRITE_ONCE(y, r2);
>
> into something resembling
>
> r1 = READ_ONCE(x);
> WRITE_ONCE(y, r2);
> if (r1) {
> ...
> }
>
> (provided the "..." part doesn't contain any volatile accesses,
> barriers, or anything affecting r2), which would destroy any run-time
> control dependency. The CPU could then execute the write before the
> read.
True, but almost all current litmus tests do have at least one volatile
access in their "if" statements. I am guessing that this has the same
memory-model tooling issues as #2 below, but I am as usual happy to be
proven wrong. ;-)
> > 2. Like #1 above, but only if something in one of the "if"'s
> > branches would prevent the compiler from reordering
> > (smp_mb(), synchronize_rcu(), value-returning non-relaxed
> > RMW atomic, ...). Easy for me to say, but I am guessing
> > that much violence would be needed to the tooling to make
> > this work. ;-)
>
> This would be my preference. But I'm afraid it isn't practical at the
> moment.
I bet that some combination of scripting and smallish modifications to
tooling could make it happen in reasonably short term. Might be more
difficult to make something more future-proof, though, agreed.
> > If I understand Alan correctly, there is not an obvious way to make
> > this change within the confines of the memory model's .bell and .cat
> > files.
>
> No way at all. It would require significant changes to herd's internal
> workings and its external interface -- my original point.
I was afraid of that. ;-)
Though truth be told, I was expecting an issue like this to crop up
sooner rather than later, so I was actually getting a bit nervous
about the fact that it had not yet shown itself...
Thanx, Paul
