Re: [Sablevm-developer] Threading support in SableVM

[Chris P. <chr...@ma...>]

Etienne Gagnon wrote:
> Etienne Gagnon wrote:
> 
>>> It fails in _svmf_enter_object_monitor().  I think Etienne needs to 
>>> look at it because he wrote the locking stuff ... I would like to 
>>> participate in debugging it but I don't know where to begin.  Should 
>>> we try, for example, removing thin locks altogether and see if that 
>>> fixes it?  Or try making some critical sections bigger?
> 
> 
> OK.  The bug has been fixed in staging.  Thanks a lot, Chris, for
> providing me (privately) with a small test case, and reporting the
> bug in the first place! :-)

No problem :)  Thanks for the fix.  In case anyone is wondering, it
was because in trying to acquire a lock, we initially read a lockword as
having no owner.  Then, a thinlock was acquired by another thread, and
we had to inflate it, but by now the initial lockword value was stale
and we were getting a NULL pointer to pass to pthread_mutex_lock() when
using the lockword to look up the owner. The solution was simply to
retry locking if the owner is NULL.

(Etienne pls. correct any mistakes ...)

However, I now have another test case (see attached), although I've only
seen this bug manifest itself on a multiprocessor. It might affect
uniprocessors in more complex scenarios.

I get the following about 5% of the time:

Exception in thread "Thread-1" java.lang.NullPointerException
    at IncrementRunnable.run (ThreadStarter.java:29)
    at java.lang.Thread.run (Thread.java:670)
    at java.lang.VMThread.callRun (VMThread.java:116)
    at java.lang.Thread.callRun (Thread.java:343)
    at java.lang.VirtualMachine.runThread (VirtualMachine.java:117)

I have an instruction trace (it took 26 tries to get it!).  All that I
can see is that the NullPointerException appears to be thrown right
after the main() method has exited ... anyway, I'll do some more
digging.

Cheers,
Chris

============ relevant instruction trace =======================

[verbose instructions: executing @0x41353ec4 ALOAD_0]
[verbose instructions: executing @0x41353ec8 DUP]
[verbose instructions: executing @0x41353ecc GETFIELD_INT]
[verbose instructions: executing @0x41353ed8 ICONST_1]
[verbose instructions: executing @0x41353edc IADD]
[verbose instructions: executing @0x41353ee0 PUTFIELD_INT]
[verbose instructions: executing @0x41353eec IINC]
[verbose instructions: executing @0x41353ef8 ILOAD_1]
[verbose instructions: executing @0x41353efc LDC_INTEGER]
[verbose instructions: executing @0x41353f04 IF_ICMPLT_CHECK]
[verbose instructions: executing @0x41353f10 RETURN]
[verbose methods: exiting  method IncrementRunnable.run()V]
[                   returning to ThreadStarter.main([Ljava/lang/String;)V]
[verbose instructions: executing @0x41353844 REPLACE]
[verbose instructions: executing @0x41353850 GOTO]
[verbose instructions: executing @0x41353734 RETURN]
[verbose methods: exiting  method ThreadStarter.main([Ljava/lang/String;)V]
[                   returning to 
java/lang/VirtualMachine.invokeMain(Ljava/lang/Class;[Ljava/lang/String;)V]
[verbose instructions: executing @0x8050eb0 INTERNAL_CALL_END]
[verbose methods: exiting  method 			 
java/lang/VirtualMachine.invokeMain(Ljava/lang/Class;[Ljava/lang/String;)V]
[                   returning to 
java/lang/VirtualMachine.main([Ljava/lang/String;)V]
[verbose instructions: executing @0x412efff4 REPLACE]
[verbose instructions: executing @0x412f0000 GOTO]
[verbose instructions: executing @0x412eff50 RETURN]
[verbose methods: exiting  method 
java/lang/VirtualMachine.main([Ljava/lang/String;)V]
[verbose instructions: executing @0x8050eb0 INTERNAL_CALL_END]
[verbose methods: entering method 
java/lang/NullPointerException.<init>(Ljava/lang/String;)V]
[verbose instructions: executing @0x41274204 ALOAD_0]
[verbose instructions: executing @0x41274208 ALOAD_1]
[verbose instructions: executing @0x4127420c INVOKESPECIAL]
[verbose methods: entering method 
java/lang/RuntimeException.<init>(Ljava/lang/String;)V]
[verbose instructions: executing @0x41273df4 ALOAD_0]
[verbose instructions: executing @0x41273df8 ALOAD_1]
[verbose instructions: executing @0x41273dfc INVOKESPECIAL]
[verbose methods: entering method 
java/lang/Exception.<init>(Ljava/lang/String;)V]
[verbose instructions: executing @0x41273e38 ALOAD_0]
[verbose instructions: executing @0x41273e3c ALOAD_1]
[verbose instructions: executing @0x41273e40 INVOKESPECIAL]
[verbose methods: entering method 
java/lang/Throwable.<init>(Ljava/lang/String;)V]
[verbose instructions: executing @0x41273e7c ALOAD_0]
[verbose instructions: executing @0x41273e80 INVOKESPECIAL]
[verbose methods: entering method java/lang/Object.<init>()V]
[verbose instructions: executing @0x41251644 RETURN]
[verbose methods: exiting  method java/lang/Object.<init>()V]
[                   returning to 
java/lang/Throwable.<init>(Ljava/lang/String;)V]
[verbose instructions: executing @0x41273e94 ALOAD_0]
[verbose instructions: executing @0x41273e98 ALOAD_0]
[verbose instructions: executing @0x41273e9c PUTFIELD_REFERENCE]

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