tree 88e05684326c5f64b1c4d128b6aa62daa75f7e4c
parent 12c8e76120c41c09d20ec0b187e10c5db967a073
author Paul E. McKenney <paulmck@linux.vnet.ibm.com> 1480454027 -0800
committer Paul E. McKenney <paulmck@linux.vnet.ibm.com> 1480489538 -0800

rcu: Add functions to test for trivial grace periods

Under some circumstances, RCU grace periods are zero cost.  For
RCU-preempt, this is the case during boot, and for RCU-bh and RCU-sched,
this is the case if there is only one CPU.  This means that RCU users
might wish to dispense with grace-period-avoidance strategies when
grace periods are zero cost, so this commit adds rcu_trivial_gp(),
rcu_bh_trivial_gp(), and rcu_sched_trivial_gp() to test for these
conditions.  Because the conditions leading to zero-cost grace periods
can change at any time (for example, when a second CPU is onlined), these
functions should be used as performance hints, and must not be relied
on for correctness.  For example, even if rcu_trivial_gp() returns true,
you are required to invoke synchronize_rcu().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>