[jdk17u] RFR: 8276205: Shenandoah: CodeCache_lock should always be held for initializing code cache iteration

[Zhengyu Gu]

I would like to backport this Shenandoah specific fix to jdk17u.

This patch fixed the bug that not holding CodeCache_lock while initializing concurrent nmethod iterations, that can lead to miscount number of concurrent walker and result assertion failure.

The patch fixed a bug that prevents iterators from walking nmethod concurrently due to holding CodeCache_lock during the walk.

The original patch does not apply cleanly, due to context changes. But the patch is small and simple, conflicts can be easily resolved manually.


Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - Backport 99b7b95e014da6e491ba7adfd21de53d6ae166fe

Changes: https://git.openjdk.java.net/jdk17u/pull/262/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=262&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276205
  Stats: 7 lines in 2 files changed: 2 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/262.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/262/head:pull/262

PR: https://git.openjdk.java.net/jdk17u/pull/262