[PATCH tip/core/rcu 07/10] rcu: Prevent dyntick-idle until ksoftirqd has been spawned

[paulmck@kernel.org]

From: "Paul E. McKenney" <paulmck@kernel.org>

After interrupts have enabled at boot but before some random point
in early_initcall() processing, softirq processing is unreliable.
If softirq sees a need to push softirq-handler invocation to ksoftirqd
during this time, then those handlers can be delayed until the ksoftirqd
kthreads have been spawned, which happens at some random point in the
early_initcall() processing.  In many cases, this delay is just fine.
However, if the boot sequence blocks waiting for a wakeup from a softirq
handler, this delay will result in a silent-hang deadlock.

This commit therefore prevents these hangs by ensuring that the tick
stays active until after the ksoftirqd kthreads have been spawned.
This change causes the tick to eventually drain the backlog of delayed
softirq handlers, breaking this deadlock.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_plugin.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2d60377..36212de 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1255,6 +1255,11 @@ static void rcu_prepare_kthreads(int cpu)
  */
 int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
+	/* Through early_initcall(), need tick for softirq handlers. */
+	if (!IS_ENABLED(CONFIG_HZ_PERIODIC) && !this_cpu_ksoftirqd()) {
+		*nextevt = 1;
+		return 1;
+	}
 	*nextevt = KTIME_MAX;
 	return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
 	       !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);
@@ -1350,6 +1355,12 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 
 	lockdep_assert_irqs_disabled();
 
+	/* Through early_initcall(), need tick for softirq handlers. */
+	if (!IS_ENABLED(CONFIG_HZ_PERIODIC) && !this_cpu_ksoftirqd()) {
+		*nextevt = 1;
+		return 1;
+	}
+
 	/* If no non-offloaded callbacks, RCU doesn't need the CPU. */
 	if (rcu_segcblist_empty(&rdp->cblist) ||
 	    rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
-- 
2.9.5