rcu: Suppress NMI backtraces when stall ends before dump

It is possible for an RCU CPU stall to end just as it is detected, in
which case the current code will uselessly dump all CPU's stacks.
This commit therefore checks for this condition and refrains from
sending needless NMIs.

And yes, the stall might also end just after we checked all CPUs and
tasks, but in that case we would at least have given some clue as
to which CPU/task was at fault.

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

1 files changed, 9 insertions, 4 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 9970116163ba..ade788320dd6 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -545,6 +545,7 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
         int cpu;
         long delta;
         unsigned long flags;
+        int ndetected;
         struct rcu_node *rnp = rcu_get_root(rsp);
 
         /* Only let one CPU complain about others per time interval. */
@@ -561,7 +562,7 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
          * Now rat on any tasks that got kicked up to the root rcu_node
          * due to CPU offlining.
          */
-        rcu_print_task_stall(rnp);
+        ndetected = rcu_print_task_stall(rnp);
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
 
         /*
@@ -573,17 +574,21 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
                rsp->name);
         rcu_for_each_leaf_node(rsp, rnp) {
                 raw_spin_lock_irqsave(&rnp->lock, flags);
-                rcu_print_task_stall(rnp);
+                ndetected += rcu_print_task_stall(rnp);
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 if (rnp->qsmask == 0)
                         continue;
                 for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
-                        if (rnp->qsmask & (1UL << cpu))
+                        if (rnp->qsmask & (1UL << cpu)) {
                                 printk(" %d", rnp->grplo + cpu);
+                                ndetected++;
+                        }
         }
         printk("} (detected by %d, t=%ld jiffies)\n",
                smp_processor_id(), (long)(jiffies - rsp->gp_start));
-        if (!trigger_all_cpu_backtrace())
+        if (ndetected == 0)
+                printk(KERN_ERR "INFO: Stall ended before state dump start\n");
+        else if (!trigger_all_cpu_backtrace())
                 dump_stack();
 
         /* If so configured, complain about tasks blocking the grace period. */