On Thu, 3 Jul 2025 03:08:26 GMT, Alex Menkov <amen...@openjdk.org> wrote:
>> Currently jvmtiAgentList keeps agents in reversed order (new agents are >> added to the head of the list). >> To restore original order JvmtiAgentList::Iterator uses GrowableArray >> allocated in heap. >> Iterators for different agent types are returned by value, and the iterator >> class nas no custom copy ctor, so if the constructor not elides, >> GrowableArray is deallocated twice. >> >> The fix updates jvmtiAgentList to keep agents in the original order, agents >> are added to the tail. >> Iterator now needs only single pointer to next agent. >> Additionally removed `JvmtiAgentList::Iterator::next() const` method (it >> looks very strange as `next()` is expected to change state of the iterator). >> >> Testing: tier1..4,hs-tier5-svc > > Alex Menkov has updated the pull request incrementally with one additional > commit since the last revision: > > jcheck Okay - still took me a little while to understand the double-indirection of the "tail ptr" in `add`, but I get it now. So looking at the acquire/release requirements: - all additions are done with `cmpxchg` which is effectively a release-store - when you iterate the list all loads of the "next" agent must be a load-acquire, this means - when you create the iterator you need a load-acquire (which you have when you pass `head()` ) - In `Iterator::next()` you need a load-acquire on each read of the `_next` field which is most simply done by defining `JvmtiAgent::next()` as a load-acquire and using that in the iterator code instead of direct field access. - `JvmtiAgent::set_next` should be a release-store though as far as I can see it is not used. ------------- Changes requested by dholmes (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/26083#pullrequestreview-2981588585
