[aarch64-port-dev ] Question about JVM option "-XX:+UseBarriersForVolatile" usage in aarch64.

[Xiaohong Gong]

Hi,

Recently we met the Renassiance benchmark hang issue on AArch64 when running with VM option "-XX:+UseBarriersForVolatile".
We noticed that it should be the same memory sequentially consistent issue mentioned in JDK-8179954<https://bugs.openjdk.java.net/browse/JDK-8179954> which is caused by mixed
using "LDR, DMB ISHLD" for java volatile load and "STLR/STLXR" for volatile store.

JDK-8179954 resolved it by inserting a full barrier before the volatile load in C1/Interpreter. However it only happens with the VM
option "UseBarriersForVolatile" closed, because C2 only uses "LDAR/STLR" to implement the volatile access under it.
Here is a part of the fixing codes in Interpreter:
       if (! UseBarriersForVolatile) {
           Label notVolatile;
           __ tbz(raw_flags, ConstantPoolCacheEntry::is_volatile_shift, notVolatile);
           __ membar(MacroAssembler::AnyAny);
           __ bind(notVolatile);
       }

However, there are also some other places that use "STLR/STLXR" in hotspot when "UseBarriersForVolatile" is true, like monitor and
atomics. For atomics like CAS, they do not have a trailing "STORE_LOAD" barrier with "-XX:+UseBarriersForVolatile". This might have
the memory consistent issue if there is a subsequent volatile load ("LDR, DMB ISHLD").
Here is the codes generated for CAS by C2 with "-XX:+UseBarriersForVolatile":
              dmb ish
        retry:
                ldxr w0, [address]
                cmp w0, w1
                b.ne done
                stlxr w8, w2, [address]
                cbnz w8, retry
        done:
                dmb ishld       // LOAD_LOAD | LOAD_STORE
Without a STORE_LOAD barrier,  the subsequent load could be float up before "stlxr" of the CAS. This might expected not to happen
if the load is a volatile one.

So my question is: what does the main usage of the option "-XX:+UseBarriersForVolatile" in real applications? Do we need to concern
the issues with it? If we do, so maybe the issue is valuable to be fixed.
I also created a JBS here: https://bugs.openjdk.java.net/browse/JDK-8241137.

Thanks,
Xiaohong Gong