rcu: Make expedited RCU-preempt stall warnings count accurately

Currently, synchronize_sched_expedited_wait() simply sets the ndetected
variable to the rcu_print_task_exp_stall() return value.  This means
that if the last rcu_node structure has no stalled tasks, record of
any stalled tasks in previous rcu_node structures is lost, which can
in turn result in failure to dump out the blocking rcu_node structures.
Or could, had the test been correct.

This commit therefore adds the return value of rcu_print_task_exp_stall()
to ndetected and corrects the later test for ndetected.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5f4336fadc28..687d8a5f35c7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3778,7 +3778,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
                        rsp->name);
                 ndetected = 0;
                 rcu_for_each_leaf_node(rsp, rnp) {
-                        ndetected = rcu_print_task_exp_stall(rnp);
+                        ndetected += rcu_print_task_exp_stall(rnp);
                         mask = 1;
                         for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
                                 struct rcu_data *rdp;
@@ -3797,7 +3797,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
                 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
                         jiffies - jiffies_start, rsp->expedited_sequence,
                         rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
-                if (!ndetected) {
+                if (ndetected) {
                         pr_err("blocking rcu_node structures:");
                         rcu_for_each_node_breadth_first(rsp, rnp) {
                                 if (rnp == rnp_root)