RFR: 8374449: Shenandoah: Leaf locks used by Shenandoah need lower ranks

[Y. Srinivas Ramakrishna]

On Wed, 21 Jan 2026 18:26:38 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahController.hpp line 47:
>> 
>>> 45:   const Mutex::Rank WAITERS_LOCK_RANK = Mutex::safepoint - 5;
>>> 46:   const Mutex::Rank CONTROL_LOCK_RANK = Mutex::nosafepoint - 2;
>>> 47: 
>> 
>> I see code where we lock gc waiters lock then proceed to take control lock, e.g.
>> 
>> https://github.com/openjdk/jdk/blob/e25a5a4821d03680d00ab6bdbec727732add8206/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp#L767
>> 
>> which calls:
>> 
>> https://github.com/openjdk/jdk/blob/e25a5a4821d03680d00ab6bdbec727732add8206/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp#L703
>> 
>> Wouldn't that mean we should rank the latter lower than the former?
>> 
>> May be I am confusing myself on the direction of locking rank. I thought that we can only take locks in reducing order of rank (so   can't take (safepoint - 2) while holding (safepoint - 5))?
>> 
>> https://github.com/openjdk/jdk/blob/e25a5a4821d03680d00ab6bdbec727732add8206/src/hotspot/share/runtime/mutex.cpp#L452
>> 
>> Can you run all of the jtreg tests to see if we are safe across all paths, or if locks need reranking?
>
> You are correct on all accounts here, except the basis for control lock is `Mutex::nosafepoint` and the basis for the waiter's locks is `Mutex::safepoint`. `safepoint` is defined as `nosafepoint + 20` so `nosafepoint` _is_ lower.  I didn't actually change the rank of the control lock, just pulled it into a shared constant as both `ShControlThread` and `ShGenerationalControlThread` share the value. This branch passed GHA and internal testing.

Thanks, sorry for my confusion and my sloppy reading of the, ahem, minuend :-)

I'll reread more carefully and approve; the ranks now make sense of course.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29333#discussion_r2714270228