pgsql: bufmgr: Make UnlockReleaseBuffer() more efficient

bufmgr: Make UnlockReleaseBuffer() more efficient

Now that the buffer content lock is implemented as part of BufferDesc.state,
releasing the lock and unpinning the buffer can be implemented as a single
atomic operation.

This improves workloads that have heavy contention on a small number of
buffers substantially, I e.g., see a ~20% improvement for pipelined readonly
pgbench on an older two socket machine.

Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://postgr.es/m/5ubipyssiju5twkb7zgqwdr7q2vhpkpmuelxfpanetlk6ofnop@hvxb4g2amb2d

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f39cb8c011062d65e146c1e9d1aae221e96d8320

Modified Files
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src/backend/storage/buffer/bufmgr.c | 58 +++++++++++++++++++++++++++++++++++--
1 file changed, 55 insertions(+), 3 deletions(-)