[PATCH tip/core/rcu] Introduce check for callback list/count mismatch

[Paul E. McKenney]

The recent bug that introduced the RCU callback list/count mismatch
showed the need for a diagnostic to check for this, which this patch
adds.

Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 95c7b61..4154c95 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1612,6 +1612,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 		rdp->n_force_qs_snap = rsp->n_force_qs;
 	} else if (rdp->qlen < rdp->qlen_last_fqs_check - qhimark)
 		rdp->qlen_last_fqs_check = rdp->qlen;
+	WARN_ON_ONCE((rdp->nxtlist == NULL) != (rdp->qlen == 0));
 
 	local_irq_restore(flags);
 

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