[PATCH rcu 4/6] rcu/tasks: Mark callbacks not currently participating in barrier operation

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

Each Tasks RCU flavor keeps a count of the number of callbacks that the
current rcu_barrier_tasks*() 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/tasks.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index cf273c554f3b0..6f1e82e548bbc 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -281,6 +281,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
 			INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
 		if (!rtpcp->rtp_exit_list.next)
 			INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
+		rtpcp->barrier_q_head.next = &rtpcp->barrier_q_head;
 		maxcpu = cpu;
 	}
 
@@ -405,6 +406,7 @@ static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
 	struct rcu_tasks *rtp;
 	struct rcu_tasks_percpu *rtpcp;
 
+	rhp->next = rhp; // Mark the callback as having been invoked.
 	rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
 	rtp = rtpcp->rtpp;
 	if (atomic_dec_and_test(&rtp->barrier_q_count))
-- 
2.40.1