MDEV-40210 Redundant CAS in async_flush_lsn::try_clear_if_at_most()

[iMineLink]

MDEV-39600 added try_clear_if_at_most() to clear buf_flush_async_lsn with an atomic CAS that preserves a concurrent bump(). If the snapshot is already 0, compare_exchange_strong(0, 0) is a no-op, so return early on a zero snapshot and avoid the atomic read-modify-write. The page cleaner calls this on every pass, so in the common steady state (no async flush queued) it drops needless exclusive access to the m_lsn cache line. A zero value is already the cleared state and a concurrent bump() is preserved either way, so the result is identical.

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[gemini-code-assist]

Code Review

This pull request introduces an optimization in storage/innobase/buf/buf0flu.cc within the async_flush_lsn::try_clear_if_at_most function. It adds a check to return early if the snapshot is already zero, thereby avoiding a redundant atomic compare-and-swap (CAS) operation. There are no review comments, and we have no feedback to provide.

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[Copilot]

Pull request overview

This PR optimizes InnoDB’s page-cleaner hot path by avoiding a redundant atomic read-modify-write when the async flush LSN is already in the cleared state (0), preserving the same concurrency semantics while reducing cache-line contention.

Changes:

• Add an early return in async_flush_lsn::try_clear_if_at_most() when the snapshotted LSN is 0 to skip a no-op CAS.
• Reduce unnecessary exclusive access to the m_lsn cache line during steady-state page-cleaner passes.

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[dr-m]

This is a trivial fixup to #5078. An added conditional branch should incur a much smaller worst-case overhead than an atomic compare-and-swap.