RFR: 8313202: MutexLocker should disallow null Mutexes

[David Holmes]

On Wed, 26 Jul 2023 17:06:02 GMT, Aleksey Shipilev <shade 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 convey intent we need to, IMO, change the naming style to something that relates to when we lock - the "FooLocker" naming doesn't work well in that regard: Conditional is okay except that "unless already locked" is  also a condition - and Reentrant is totally wrong for that case.

Something like: 
- MutexLockWhen for the general predicate case
- MutexLockIfNeeded for the already locked (or safepoint?) case.

To be clear I very strongly object to ReentrantMutexLocker as a name.

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PR Comment: https://git.openjdk.org/jdk/pull/15043#issuecomment-1654682294
PR Comment: https://git.openjdk.org/jdk/pull/15043#issuecomment-1654683573