• Add sysctls at debug.rwlock to control the behavior of the speculative spinning when readers hold a lock. This spinning is speculative because, unlike the write case, we can not test whether the owners are running.
• Add speculative read spinning for readers who are blocked by pending writers while a read lock is still held. This allows the thread to spin until the write lock succeeds after which it may spin until the writer has released the lock. This prevents excessive context switches when readers and writers both hold the lock for brief periods.

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