On Mon, Jan 25, 2016 at 06:02:34PM +0000, Will Deacon wrote:
> Hi Paul,
>
> On Fri, Jan 15, 2016 at 09:39:12AM -0800, Paul E. McKenney wrote:
> > On Fri, Jan 15, 2016 at 09:55:54AM +0100, Peter Zijlstra wrote:
> > > On Thu, Jan 14, 2016 at 01:29:13PM -0800, Paul E. McKenney wrote:
> > > > So smp_mb() provides transitivity, as do pairs of smp_store_release()
> > > > and smp_read_acquire(),
> > >
> > > But they provide different grades of transitivity, which is where all
> > > the confusion lays.
> > >
> > > smp_mb() is strongly/globally transitive, all CPUs will agree on the order.
> > >
> > > Whereas the RCpc release+acquire is weakly so, only the two cpus
> > > involved in the handover will agree on the order.
> >
> > Good point!
> >
> > Using grace periods in place of smp_mb() also provides strong/global
> > transitivity, but also insanely high latencies. ;-)
> >
> > The patch below updates Documentation/memory-barriers.txt to define
> > local vs. global transitivity. The corresponding ppcmem litmus test
> > is included below as well.
> >
> > Should we start putting litmus tests for the various examples
> > somewhere, perhaps in a litmus-tests directory within each participating
> > architecture? I have a pile of powerpc-related litmus tests on my laptop,
> > but they probably aren't doing all that much good there.
>
> I too would like to have the litmus tests in the kernel so that we can
> refer to them from memory-barriers.txt. Ideally they wouldn't be targetted
> to a particular arch, however.
Agreed. Working on it...
> > PPC local-transitive
> > ""
> > {
> > 0:r1=1; 0:r2=u; 0:r3=v; 0:r4=x; 0:r5=y; 0:r6=z;
> > 1:r1=1; 1:r2=u; 1:r3=v; 1:r4=x; 1:r5=y; 1:r6=z;
> > 2:r1=1; 2:r2=u; 2:r3=v; 2:r4=x; 2:r5=y; 2:r6=z;
> > 3:r1=1; 3:r2=u; 3:r3=v; 3:r4=x; 3:r5=y; 3:r6=z;
> > }
> > P0 | P1 | P2 | P3 ;
> > lwz r9,0(r4) | lwz r9,0(r5) | lwz r9,0(r6) | stw r1,0(r3) ;
> > lwsync | lwsync | lwsync | sync ;
> > stw r1,0(r2) | lwz r8,0(r3) | stw r1,0(r7) | lwz r9,0(r2) ;
> > lwsync | lwz r7,0(r2) | | ;
> > stw r1,0(r5) | lwsync | | ;
> > | stw r1,0(r6) | | ;
> > exists
> > (* (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r8=0 /\ 3:r9=0) *)
> > (* (0:r9=1 /\ 1:r9=1 /\ 2:r9=1) *)
> > (* (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r7=0) *)
> > (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r7=0)
>
> i.e. we should rewrite this using READ_ONCE/WRITE_ONCE and smp_mb() etc.
Yep!
> > ------------------------------------------------------------------------
> >
> > commit 2cb4e83a1b5c89c8e39b8a64bd89269d05913e41
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Date: Fri Jan 15 09:30:42 2016 -0800
> >
> > documentation: Distinguish between local and global transitivity
> >
> > The introduction of smp_load_acquire() and smp_store_release() had
> > the side effect of introducing a weaker notion of transitivity:
> > The transitivity of full smp_mb() barriers is global, but that
> > of smp_store_release()/smp_load_acquire() chains is local. This
> > commit therefore introduces the notion of local transitivity and
> > gives an example.
> >
> > Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Reported-by: Will Deacon <will.deacon@xxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index c66ba46d8079..d8109ed99342 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1318,8 +1318,82 @@ or a level of cache, CPU 2 might have early access to CPU 1's writes.
> > General barriers are therefore required to ensure that all CPUs agree
> > on the combined order of CPU 1's and CPU 2's accesses.
> >
> > -To reiterate, if your code requires transitivity, use general barriers
> > -throughout.
> > +General barriers provide "global transitivity", so that all CPUs will
> > +agree on the order of operations. In contrast, a chain of release-acquire
> > +pairs provides only "local transitivity", so that only those CPUs on
> > +the chain are guaranteed to agree on the combined order of the accesses.
>
> Thanks for having a go at this. I tried defining something axiomatically,
> but got stuck pretty quickly. In my scheme, I used "data-directed
> transitivity" instead of "local transitivity", since the latter seems to
> be a bit of a misnomer.
I figured that "local" meant local to the CPUs participating in the
release-acquire chain. As opposed to smp_mb() chains where the ordering
is "global" as in visible to all CPUs, whether on the chain or not.
Does that help?
> > +For example, switching to C code in deference to Herman Hollerith:
> > +
> > + int u, v, x, y, z;
> > +
> > + void cpu0(void)
> > + {
> > + r0 = smp_load_acquire(&x);
> > + WRITE_ONCE(u, 1);
> > + smp_store_release(&y, 1);
> > + }
> > +
> > + void cpu1(void)
> > + {
> > + r1 = smp_load_acquire(&y);
> > + r4 = READ_ONCE(v);
> > + r5 = READ_ONCE(u);
> > + smp_store_release(&z, 1);
> > + }
> > +
> > + void cpu2(void)
> > + {
> > + r2 = smp_load_acquire(&z);
> > + smp_store_release(&x, 1);
> > + }
> > +
> > + void cpu3(void)
> > + {
> > + WRITE_ONCE(v, 1);
> > + smp_mb();
> > + r3 = READ_ONCE(u);
> > + }
> > +
> > +Because cpu0(), cpu1(), and cpu2() participate in a local transitive
> > +chain of smp_store_release()/smp_load_acquire() pairs, the following
> > +outcome is prohibited:
> > +
> > + r0 == 1 && r1 == 1 && r2 == 1
> > +
> > +Furthermore, because of the release-acquire relationship between cpu0()
> > +and cpu1(), cpu1() must see cpu0()'s writes, so that the following
> > +outcome is prohibited:
> > +
> > + r1 == 1 && r5 == 0
> > +
> > +However, the transitivity of release-acquire is local to the participating
> > +CPUs and does not apply to cpu3(). Therefore, the following outcome
> > +is possible:
> > +
> > + r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
>
> I think you should be completely explicit and include r5 == 1 here, too.
Good point -- I added this as an additional outcome:
r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
> Also -- where would you add the smp_mb__after_release_acquire to fix
> (i.e. forbid) this? Immediately after cpu1()'s read of y?
That sounds plausible, but we would first have to agree on exactly
what smp_mb__after_release_acquire() did. ;-)
> > +Although cpu0(), cpu1(), and cpu2() will see their respective reads and
> > +writes in order, CPUs not involved in the release-acquire chain might
> > +well disagree on the order. This disagreement stems from the fact that
> > +the weak memory-barrier instructions used to implement smp_load_acquire()
> > +and smp_store_release() are not required to order prior stores against
> > +subsequent loads in all cases. This means that cpu3() can see cpu0()'s
> > +store to u as happening -after- cpu1()'s load from v, even though
> > +both cpu0() and cpu1() agree that these two operations occurred in the
> > +intended order.
> > +
> > +However, please keep in mind that smp_load_acquire() is not magic.
> > +In particular, it simply reads from its argument with ordering. It does
> > +-not- ensure that any particular value will be read. Therefore, the
> > +following outcome is possible:
> > +
> > + r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0
> > +
> > +Note that this outcome can happen even on a mythical sequentially
> > +consistent system where nothing is ever reordered.
>
> I'm not sure this last bit is strictly necessary. If somebody thinks that
> acquire/release involve some sort of implicit synchronisation, I think
> they may have bigger problems with memory-barriers.txt.
Agreed. But unless I add text like this occasionally, such people could
easily read through much of memory-barriers.txt and think that they did
in fact understand it. So I have to occasionally trip an assertion in
their brain. Or try to... :-/
Thanx, Paul
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