rcu: Prevent rcu_barrier() from starting needless grace periods
Currently rcu_barrier() uses call_rcu() to enqueue new callbacks
on each CPU with a non-empty callback list. This works, but means
that rcu_barrier() forces grace periods that are not otherwise needed.
The key point is that rcu_barrier() never needs to wait for a grace
period, but instead only for all pre-existing callbacks to be invoked.
This means that rcu_barrier()'s new callbacks should be placed in
the callback-list segment containing the last pre-existing callback.
This commit makes this change using the new rcu_segcblist_entrain()
function.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e354e47..657056c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3578,8 +3578,14 @@ static void rcu_barrier_func(void *type)
struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
_rcu_barrier_trace(rsp, "IRQ", -1, rsp->barrier_sequence);
- atomic_inc(&rsp->barrier_cpu_count);
- rsp->call(&rdp->barrier_head, rcu_barrier_callback);
+ rdp->barrier_head.func = rcu_barrier_callback;
+ debug_rcu_head_queue(&rdp->barrier_head);
+ if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head, 0)) {
+ atomic_inc(&rsp->barrier_cpu_count);
+ } else {
+ debug_rcu_head_unqueue(&rdp->barrier_head);
+ _rcu_barrier_trace(rsp, "IRQNQ", -1, rsp->barrier_sequence);
+ }
}
/*