RFR: 8274282: Clarify special wait assert [v2]

[David Holmes]

On Fri, 24 Sep 2021 16:59:18 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> This change removes the assert and tests for Mutex::wait() only allowed with greater than nosafepoint ranked held locks.
>> Tested with tier1-3.
>
> Coleen Phillimore has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
> 
>   Fix wait checking code.

Hi Coleen,

The rationale is not quite right to me here. See comments below.

Thanks,
David

src/hotspot/share/runtime/mutex.cpp line 388:

> 386:     Mutex* least = get_least_ranked_lock_besides_this(locks_owned);
> 387:     // We enforce not holding locks of rank nosafepoint or lower while waiting for JavaThreads,
> 388:     // because the held lock has a NoSafepointVerifier so waiting on a lock will block out safepoints.

The NSV is not the reason, it is  a mechanism to detect violation of the rules. The reason we must not block in a JavaThread is because it will block without a safepoint check and without becoming safepoint-safe, thus blocking out safepoints while the wait is in progress.

Also note that this applies to any wait() on a nosafepoint lock, not just one done while holding a safepoint lock.

src/hotspot/share/runtime/mutex.cpp line 389:

> 387:     // We enforce not holding locks of rank nosafepoint or lower while waiting for JavaThreads,
> 388:     // because the held lock has a NoSafepointVerifier so waiting on a lock will block out safepoints.
> 389:     // For NonJavaThreads, we enforce not waiting on the tty lock since that could block progress also.

You are checking this for all threads not just NJTs. Is the comment wrong or the code?

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PR: https://git.openjdk.java.net/jdk/pull/5684