D.2.7.1 Start a New Grace Period
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D.2.7.1 Start a New Grace Period

The rcu_start_gp() function starts a new grace period. This function is invoked when a CPU having callbacks waiting for a grace period notices that no grace period is in progress.

The rcu_start_gp() function updates state in the rcu_state and rcu_data structures to note the newly started grace period, acquires the ->onoff lock (and disables irqs) to exclude any concurrent CPU-hotplug operations, sets the bits in all of the rcu_node structures to indicate that all CPUs (including this one) must pass through a quiescent state, and finally releases the ->onoff lock.

The bit-setting operation is carried out in two phases. First, the non-leaf rcu_node structures' bits are set without holding any additional locks, and then finally each leaf rcu_node structure's bits are set in turn while holding that structure's ->lock.

Quick Quiz D.11: But what happens if a CPU tries to report going through a quiescent state (by clearing its bit) before the bit-setting CPU has finished? End Quick Quiz

Quick Quiz D.12: And what happens if all CPUs try to report going through a quiescent state before the bit-setting CPU has finished, thus ending the new grace period before it starts? End Quick Quiz