RFR: 8212663: Remove conservative at_safepoint assert when JFR writes type sets during class unloading (original) (raw)

David Holmes david.holmes at oracle.com
Fri Oct 19 07:52:21 UTC 2018

Hi Erik,

On 19/10/2018 5:25 PM, Erik Österlund wrote:

Hi David,

Isn't the single thread call context the assumed default?

Not sure what you mean.

In particular, this is called by SystemDictionary::dounloading() only. And none of that stuff would work if called by multiple threads. So while it is indeed possible to distinguish the ZDriver thread as a special type of concurrent GC thread that also handles unloading, and verify there can only be one such thread with that role, for this assert, I'm not sure if it is worth the hassle as all code in the unloading path is single threaded anyway, and I can't see what is special about this code apart from that there was an assert there before and the other single threaded code did not. Do you agree?

The author of the current code considered it desriable to check the code was only called at a safepoint. That might mean it only executes in the VMThread or it might not - the single-threaded aspect of this code is not evident to me just from the code. If checking for a safepoint was really a proxy for checking it was executed by the VMThread, and concurrent unloading now uses a different thread, then it didn't seem unreasonable to ask if the assertion could be modified to cover that rather than just removed.

If it's really too much trouble than that's fine, but it was worth asking the question.

Cheers, David

Thanks, /Erik

On 2018-10-19 03:51, David Holmes wrote: Hi Erik,

On 19/10/2018 1:00 AM, Erik Österlund wrote: Hi,

JFR writes type sets during class unloading. It currently asserts this is done in a safepoint. But in fact, it suffices that it is done by a single thread. This assert needs to be relaxed for concurrent class unloading. Okay but you removed it completely. Is there not something you can assert to verify your single thread requirement: Thread::current()->isVMthread() || Thread::current()->is classUnloaderthread() ?? Cheers, David Webrev: http://cr.openjdk.java.net/~eosterlund/8212663/webrev.00/ Bug: https://bugs.openjdk.java.net/browse/JDK-8212663 Thanks, /Erik

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