RFR(m): 8220351: Cross-modifying code

[Andrew Haley]

On 3/8/19 3:24 PM, Robbin Ehn wrote:
> Hi all, please review.
> 
> Issue:
> https://bugs.openjdk.java.net/browse/JDK-8220351
> Changeset:
> http://cr.openjdk.java.net/~rehn/8220351/webrev/
> 
> After a JavaThread have been in a safepoint/(handshake) safe state it can start
> executing updated code. E.g. an oop in the instruction stream can have been
> updated.
> 
> Most hardware's require a barrier or that the code cross modified is far away to
> guarantee that the thread executing the updated instruction stream sees the
> modification.
> 
> What far away is and how far an update instruction stream is from a safepoint
> safe state is not clear.
> 
> To be compliant with those architectures an instruction stream barrier must be
> added when leaving the safepoint safe state.
> 
> There may be crashes today due to this missing barrier.
> A new CPU with deeper pipeline or changes to the VM which moves a safepoint safe
> state closer to a nmethod can increase changes of a crash.

What is this hunk for?

@@ -2558,12 +2558,9 @@
   //     VM thread changes sync state to synchronizing and suspends threads for GC.
   //     Thread A is resumed to finish this native method, but doesn't block here since it
   //     didn't see any synchronization is progress, and escapes.
-  __ movl(Address(r15_thread, JavaThread::thread_state_offset()), _thread_in_native_trans);
-
-  // Force this write out before the read below
-  __ membar(Assembler::Membar_mask_bits(
-              Assembler::LoadLoad | Assembler::LoadStore |
-              Assembler::StoreLoad | Assembler::StoreStore));
+
+  __ lock();
+  __ addl(Address(r15_thread, JavaThread::thread_state_offset()), 1 /* _thread_in_native_trans */);

   Label after_transition;



-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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