[aarch64-port-dev ] RFR: [8u] 8209835: Aarch64: elide barriers on all volatile operations

[Yangfei (Felix)]

Ping...
Is that enough for us to push the 8u backport patch ? 
It still applies: http://cr.openjdk.java.net/~fyang/8209835-8u-backport/webrev.00/

Thanks,
Felix


> -----Original Message-----
> From: Yangfei (Felix)
> Sent: Friday, October 11, 2019 11:41 AM
> To: 'Andrew Haley' <aph at redhat.com>; aarch64-port-dev at openjdk.java.net
> Subject: RE: [aarch64-port-dev ] RFR: [8u] 8209835: Aarch64: elide barriers on
> all volatile operations
> 
> > On 8/28/19 11:39 AM, Yangfei (Felix) wrote:
> > >> Having said that, I can sort-of imagine a jcstress failure if we
> > >> had a test that
> > was doing compareAndSwap in one thread and getAndSet in another. That
> > would be a seriously compelling reason for a backport!
> > > Can you elaborate more please? especially the possible code sequence.
> > >
> > > Currently, we have C2 JIT for getAndSet like this:
> > >
> > >  33   ;; membar_release
> > >  34   0x0000ffffa416ae74: dmb       ish
> > >  35   ;; 0x40000000000
> > >  36   0x0000ffffa416ae78: orr       x11, xzr, #0x40000000000
> > >  37   0x0000ffffa416ae7c: add       x10, x1, #0x10
> > >  38   0x0000ffffa416ae80: prfm      pstl1strm, [x10]
> > >  39   0x0000ffffa416ae84: ldxr      x9, [x10]
> > >  40   0x0000ffffa416ae88: stxr      w8, x11, [x10]
> > >  41   0x0000ffffa416ae8c: cbnz      w8, 0x0000ffffa416ae84
> > >  42   0x0000ffffa416ae90: mov       x10, x9
> > >  43   ;; membar_acquire
> > >  44   0x0000ffffa416ae94: dmb       ishld           ;*invokevirtual
> > getAndSetLong
> >
> > The important thing is that there should be a StoreLoad barrier
> > between a volatile store and a volatile load. If we have a getAndSet
> > followed by a CompareAndExchange that might not happen.
> >
> 
> Hi,
> 
>   Sorry for the late reply.
>   After some searching in jcstress code, I think the following test might be the
> test you wanted to see.
> 
> 
> org.openjdk.jcstress.tests.atomics.longs.AtomicLongArrayPairwiseTests.GetA
> ndSet_CAS
> 
>  43     @State
>  44     public static class S {
>  45         public final int SIZE = 256; // double the maximum cache line
>  46         public final AtomicLongArray a = new AtomicLongArray(SIZE);
>  47         public final int idx = ThreadLocalRandom.current().nextInt(SIZE);
>  48     }
> 
> 445     @JCStressTest
> 446     @JCStressMeta(G.class)
> 447     @Outcome(id = "0, 10",  expect = Expect.ACCEPTABLE, desc = "T1 ->
> T2 execution")
> 448     @Outcome(id = "20, 20", expect = Expect.ACCEPTABLE, desc = "T2 ->
> T1 execution")
> 449     public static class GetAndSet_CAS {
> 450         @Actor public void actor1(S s, LongResult2 r) { r.r1 =
> s.a.getAndSet(s.idx, 5); }
> 451         @Actor public void actor2(S s, LongResult2 r) { r.r2 =
> s.a.compareAndSet(s.idx, 0, 20) ? 20 : 10; }
> 452     }
> 
>   AtomicLongArray.getAndSet calls unsafe.getAndSetLong and
> AtomicLongArray.compareAndSet calls compareAndSetRaw which calls
> unsafe.compareAndSwapLong.
>   For this test, one thread will do actor1 method. And another thread will do
> actor2 method concurrently.
> 
> Thanks,
> Felix