Description:

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LOCK_grant is a rwlock:

  wrlock when modify privileges

  rdlock when check privileges

The fact is that we usually check priv at most scenarios, but has serious conflict.

Solution:

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So we split the LOCK_grant into an array of rwlock:

  Require one rdlock when checking priv through thread_id % array_size.

  Require all wrlock when modify priv.

It will decrease the rdlock conflict substantially.