D.4.2.2.3 rcu_flipctr
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D.4.2.2.3 `rcu_flipctr`

As noted earlier, the rcu_flipctr per-CPU array of counters contains the counter pairs that track outstanding RCU read-side critical sections. Any given counter in this array can go negative, for example, when a task is migrated to a different CPU in the middle of an RCU read-side critical section. However, the sum of the counters will still remain positive throughout the corresponding grace period, and will furthermore go to zero at the end of that grace period.