RFR: 8266742: Check W^X state on possible safepoint

[Anton Kozlov]

Hi,

Please review a check for W^X mode at safepoint. As described in the bug, this explicit check aims to catch the wrong W^X mode that may lead to a crash (such crashes are rather intermittent).

This check helped to verify the fix for https://bugs.openjdk.java.net/browse/JDK-8265292 and should help to discover similar issues. It is disabled by default to avoid unexpected failures in the regular use, but eventually it should be turned on unconditionally.

I still owe a complete W^X approach description. Hope this patch does not depend much on that.

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Commit messages:
 - Check W^X at safepoint

Changes: https://git.openjdk.java.net/jdk/pull/3920/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3920&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266742
  Stats: 24 lines in 3 files changed: 17 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3920.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3920/head:pull/3920

PR: https://git.openjdk.java.net/jdk/pull/3920