Re: [PATCH] bufmgr: tighten LWLock:BufferMapping on InvalidateBuffer

Hi Alexandre,

> If this seems acceptable, the next steps could be ...

I don't think this design works.

Rooms change the semantics of an LWLock: exclusive access no longer
excludes all other holders. Data access needs to be reworked to support
this. The buffer mapping access in this patch already has a problem:
different rooms in the same partition can map to the same hash bucket,
allowing concurrent modifications to its chain and causing entries to be
lost.

In 0001, releasing the last room while LW_FLAG_WAKE_IN_PROGRESS is set
leaves LW_EXCLUSIVE set. Shared waiters cannot clear it and remain
blocked forever unless a subsequent exclusive acquisition and release
clears it. This also affects ordinary LW_EXCLUSIVE acquisitions.

All 26 rooms share one state word. Acquirers of disjoint rooms still
CAS on the same cache line, so the scalability bottleneck remains and
may even get worse. In this case, I would increase NUM_BUFFER_PARTITIONS.

Best regards,
Yuhang Qiu