[11] RFR(S): 8209670: CompilerThread releases code buffer in destructor is unsafe

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8209670

Since JDK-8198756 [1], compiler threads can be added and removed at runtime. C1 compiler threads use
a temporary code buffer that is allocated once the thread is created and needs to be freed if the
thread is removed to avoid a memory leak (see JDK-8205499 [2]). Freeing is currently done in the
thread destructor after the thread was removed from the thread list. This means that although we are
holding the CodeCache_lock, freeing of the code blob might interfere with non-locking safepoint code
that is concurrently walking the code cache.

The fix is to free the temporary buffers at the two places where compiler threads are removed:
http://cr.openjdk.java.net/~thartmann/8209670/webrev.00/

This should be safe because it partially restores the code to pre-JDK-8205499. Testing on
hs-tier1-3, jdk-tier1-3 and hs-precheckin-comp passed.

An alternative would be to disable UseDynamicNumberOfCompilerThreads and do a proper fix in JDK 12:
http://cr.openjdk.java.net/~thartmann/8209670/webrev.01/

Thanks,
Tobias

[1] https://bugs.openjdk.java.net/browse/JDK-8198756
[2] https://bugs.openjdk.java.net/browse/JDK-8205499