Why can't we free the old data structure immediately after the pointer flip?
A reader may still hold a pointer to the old data and be mid-read. We cannot know instantly which readers are done. RCU waits for a grace period—a CPU has passed through a quiescent state (context switch or RCU unlock)—proving the reader is no longer holding the old pointer.
Answered in
How Linux RCU Unlocks Read-Side ScalingRCU lets millions of readers run lock-free while a single writer updates state by copying, mutating off-to-the-side, and atomically flipping a pointer.
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