What is a quiescent state and why does it matter in RCU?
A quiescent state is a point where a CPU is guaranteed not to be in a read-side critical section. Common quiescent states are context switches and RCU unlock calls. The grace period ends when every CPU has hit a quiescent state at least once, guaranteeing no reader still holds the old data.
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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