Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

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.

                                                        Thanx, Paul

------------------------------------------------------------------------

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)

------------------------------------------------------------------------

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.
+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
+
+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.
+
+To reiterate, if your code requires global transitivity, use general
+barriers throughout.
 
 
 ========================


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