rcu: RCU_FAST_NO_HZ must check RCU dyntick state

The current version of RCU_FAST_NO_HZ reproduces the old CLASSIC_RCU
dyntick-idle bug, as it fails to detect CPUs that have interrupted
or NMIed out of dyntick-idle mode.  Fix this by making rcu_needs_cpu()
check the state in the per-CPU rcu_dynticks variables, thus correctly
detecting the dyntick-idle state from an RCU perspective.

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

1 files changed, 10 insertions, 2 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 0ae2339ab04d..9b18227e86e8 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1051,6 +1051,8 @@ static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
 int rcu_needs_cpu(int cpu)
 {
         int c = 0;
+        int snap;
+        int snap_nmi;
         int thatcpu;
 
         /* Check for being in the holdoff period. */
@@ -1058,12 +1060,18 @@ int rcu_needs_cpu(int cpu)
                 return rcu_needs_cpu_quick_check(cpu);
 
         /* Don't bother unless we are the last non-dyntick-idle CPU. */
-        for_each_cpu_not(thatcpu, nohz_cpu_mask)
-                if (cpu_online(thatcpu) && thatcpu != cpu) {
+        for_each_online_cpu(thatcpu) {
+                if (thatcpu == cpu)
+                        continue;
+                snap = per_cpu(rcu_dynticks, thatcpu)->dynticks;
+                snap_nmi = per_cpu(rcu_dynticks, thatcpu)->dynticks_nmi;
+                smp_mb(); /* Order sampling of snap with end of grace period. */
+                if (((snap & 0x1) != 0) || ((snap_nmi & 0x1) != 0)) {
                         per_cpu(rcu_dyntick_drain, cpu) = 0;
                         per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
                         return rcu_needs_cpu_quick_check(cpu);
                 }
+        }
 
         /* Check and update the rcu_dyntick_drain sequencing. */
         if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {