rcu: Reduce synchronize_rcu_expedited() latency
The synchronize_rcu_expedited() function disables interrupts across a scan of all leaf rcu_node structures, which is not good for real-time scheduling latency on large systems (hundreds or especially thousands of CPUs). This commit therefore holds off CPU-hotplug operations using get_online_cpus(), and removes the prior acquisiion of the ->onofflock (which required disabling interrupts). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
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