[PATCH rcu 04/11] rcu: Mark callbacks not currently participating in barrier operation

["Paul E. McKenney" <paulmck@kernel.org>]

RCU keeps a count of the number of callbacks that the current
rcu_barrier() is waiting on, but there is currently no easy way to
work out which callback is stuck.  One way to do this is to mark idle
RCU-barrier callbacks by making the ->next pointer point to the callback
itself, and this commit does just that.

Later commits will use this for debug output.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 77b5b39e19a80..930846f06bee5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4383,6 +4383,7 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
 {
 	unsigned long __maybe_unused s = rcu_state.barrier_sequence;
 
+	rhp->next = rhp; // Mark the callback as having been invoked.
 	if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) {
 		rcu_barrier_trace(TPS("LastCB"), -1, s);
 		complete(&rcu_state.barrier_completion);
@@ -5404,6 +5405,8 @@ static void __init rcu_init_one(void)
 		while (i > rnp->grphi)
 			rnp++;
 		per_cpu_ptr(&rcu_data, i)->mynode = rnp;
+		per_cpu_ptr(&rcu_data, i)->barrier_head.next =
+			&per_cpu_ptr(&rcu_data, i)->barrier_head;
 		rcu_boot_init_percpu_data(i);
 	}
 }
-- 
2.40.1