Forcing initialization of string concat INDY expressions

[Aleksey Shipilev]

Hi,

On 03/01/2017 07:36 AM, David Holmes wrote:
> The INDY-fication of string concatenation has triggered a problem where a JVM TI
> agent's monitor-wait/ed callback hits an error path that uses string concat
> which triggers a mass of indy related initialization, which in turn hits monitor
> use in MethodType$ConcurrentWeakInternSet.get, which causes the VM monitor
> subsystem to be re-entered (and it is not reentrant!) so we crash. (log extract
> below - the amount of code to process this is truly scary!)

Ouch. This is an unexpected circularity. It is unusual to see Java thread to do
Java stuff when doing Object.wait.

> I assume I can cover the exact case above by replicating it? But can I
> generalize it to cover arbitrary string concat expressions that might arise on
> other error paths?

Yes, the StringConcatFactory code is deliberately lazy. If you want to link
eagerly, you would need to match the concat shape exactly (number and type of
arguments) -- probably by using the same concat code the test failed on. We can
probably eagerly link some popular concat shapes early during system init, but
that requires JDK changes.

Looking at stack trace, it seems to be generic failure when adding new
MethodType to the MethodType cache. This is not the first time that cache
participates in circularities (I wonder if it *always* does). But I am puzzled
how does this happen:

...
V  [jvm.dll+0x2b5412]  Runtime1::monitorenter+0x1e2;;
?monitorenter at Runtime1@@CAXPAVJavaThread@@PAVoopDesc@@PAVBasicObjectLock@@@Z+0x1e2
v  ~RuntimeStub::monitorenter_nofpu Runtime1 stub
J 192 c1
java.lang.invoke.MethodType$ConcurrentWeakInternSet.get(Ljava/lang/Object;)Ljava/lang/Object;
java.base at 9-internal (54 bytes) @ 0x0242d7da [0x0242d4c0+0x0000031a]
...

It calls RuntimeStub::monitorenter_nofpu destructor? Why it ends up calling into
monitorenter? There are no locks in j.l.i.MT$CWIS.get (I checked the bytecode too).

If you want a nuclear option for your test, you may want to pass
-XDstringConcat:inline to javac to disable indy string concat to bypass that
circularity completely.

Thanks,
-Aleksey