srcu: Mark callbacks not currently participating in barrier operation

SRCU keeps a count of the number of callbacks that the current
srcu_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
SRCU-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>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>

kernel/rcu/srcutree.c

@@ -137,6 +137,7 @@ static void init_srcu_struct_data(struct srcu_struct *ssp)
 		sdp->srcu_cblist_invoking = false;
 		sdp->srcu_gp_seq_needed = ssp->srcu_sup->srcu_gp_seq;
 		sdp->srcu_gp_seq_needed_exp = ssp->srcu_sup->srcu_gp_seq;
+		sdp->srcu_barrier_head.next = &sdp->srcu_barrier_head;
 		sdp->mynode = NULL;
 		sdp->cpu = cpu;
 		INIT_WORK(&sdp->work, srcu_invoke_callbacks);
@@ -1562,6 +1563,7 @@ static void srcu_barrier_cb(struct rcu_head *rhp)
 	struct srcu_data *sdp;
 	struct srcu_struct *ssp;
 
+	rhp->next = rhp; // Mark the callback as having been invoked.
 	sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
 	ssp = sdp->ssp;
 	if (atomic_dec_and_test(&ssp->srcu_sup->srcu_barrier_cpu_cnt))