RFR: 8313202: MutexLocker should disallow null Mutexes

[Daniel D. Daugherty]

On Thu, 27 Jul 2023 22:52:53 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> As seen in [JDK-8313081](https://bugs.openjdk.org/browse/JDK-8313081), it is fairly easy to pass nullptr `Mutex` to `MutexLocker` by accident, which would just silently avoid the lock.
>> 
>> There are a few places in Hotspot where we pass `nullptr` to simulate re-entrancy and/or conditionally take the lock. Those places can be more explicit, and the default `MutexLocker` can disallow nullptrs for extra safety.
>> 
>> Open for some bikeshedding on the names of the new `MutexLockers`. Particularly `ReentrantMutexLocker` might lull readers into believing it does safepoint checks on re-entrant "lock", which it actually does not do.
>> 
>> More thorough testing with different GC/JIT combinations is running now, we might find more issues there. Meanwhile, please comment on the approach.
>> 
>> Additional testing:
>>  - [x] `grep -R "MutexLocker " src/hotspot | grep -i null`, no hits
>>  - [x] `grep -R "MutexLocker " src/hotspot | grep -i ?`, no hits
>>  - [x] Linux AArch64 fastdebug, `tier1 tier2 tier3` (re-run in progress)
>
> To be clear I very strongly object to ReentrantMutexLocker as a name.

@dholmes-ora - Thanks for your comments about ReentrantMutexLocker. I couldn't
figure out a good way to write why that name was bugging me since Mutexs are not
reentrant.

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PR Comment: https://git.openjdk.org/jdk/pull/15043#issuecomment-1655978790