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Subject[PATCH tip/core/rcu 26/48] rcu: RCU_FAST_NO_HZ must check RCU dyntick state
Date
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>
---
kernel/rcutree_plugin.h | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 5c599e8..92e5c9b 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1053,6 +1053,8 @@ 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. */
@@ -1060,12 +1062,18 @@ 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) {
--
1.7.0


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