From 4bb3f9983975f5f2ca73d8e9fd18e9ba57ad2796 Mon Sep 17 00:00:00 2001
Message-Id: <4bb3f9983975f5f2ca73d8e9fd18e9ba57ad2796.1594742966.git.zanussi@kernel.org>
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References: <832f7d97d6b989a5b4860dd2dbec58ad6ad5ab81.1594742966.git.zanussi@kernel.org>
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date: Wed, 5 Oct 2011 11:45:18 -0700
Subject: [PATCH 159/329] rcu: Make ksoftirqd do RCU quiescent states
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.132-rt59.tar.xz

Implementing RCU-bh in terms of RCU-preempt makes the system vulnerable
to network-based denial-of-service attacks.  This patch therefore
makes __do_softirq() invoke rcu_bh_qs(), but only when __do_softirq()
is running in ksoftirqd context.  A wrapper layer in interposed so that
other calls to __do_softirq() avoid invoking rcu_bh_qs().  The underlying
function __do_softirq_common() does the actual work.

The reason that rcu_bh_qs() is bad in these non-ksoftirqd contexts is
that there might be a local_bh_enable() inside an RCU-preempt read-side
critical section.  This local_bh_enable() can invoke __do_softirq()
directly, so if __do_softirq() were to invoke rcu_bh_qs() (which just
calls rcu_preempt_qs() in the PREEMPT_RT_FULL case), there would be
an illegal RCU-preempt quiescent state in the middle of an RCU-preempt
read-side critical section.  Therefore, quiescent states can only happen
in cases where __do_softirq() is invoked directly from ksoftirqd.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20111005184518.GA21601@linux.vnet.ibm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/rcu/tree.c        | 18 +++++++++++++-----
 kernel/rcu/tree_plugin.h |  8 +++++++-
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1456a3d97971..1a40e3d44cb8 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -244,7 +244,19 @@ void rcu_sched_qs(void)
 			   this_cpu_ptr(&rcu_sched_data), true);
 }
 
-#ifndef CONFIG_PREEMPT_RT_FULL
+#ifdef CONFIG_PREEMPT_RT_FULL
+static void rcu_preempt_qs(void);
+
+void rcu_bh_qs(void)
+{
+	unsigned long flags;
+
+	/* Callers to this function, rcu_preempt_qs(), must disable irqs. */
+	local_irq_save(flags);
+	rcu_preempt_qs();
+	local_irq_restore(flags);
+}
+#else
 void rcu_bh_qs(void)
 {
 	RCU_LOCKDEP_WARN(preemptible(), "rcu_bh_qs() invoked with preemption enabled!!!");
@@ -255,10 +267,6 @@ void rcu_bh_qs(void)
 		__this_cpu_write(rcu_bh_data.cpu_no_qs.b.norm, false);
 	}
 }
-#else
-void rcu_bh_qs(void)
-{
-}
 #endif
 
 /*
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e9ce51e19e87..938e64c69d18 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -29,6 +29,7 @@
 #include <linux/oom.h>
 #include <linux/sched/debug.h>
 #include <linux/smpboot.h>
+#include <linux/jiffies.h>
 #include <linux/sched/isolation.h>
 #include <uapi/linux/sched/types.h>
 #include "../time/tick-internal.h"
@@ -1408,7 +1409,7 @@ static void rcu_prepare_kthreads(int cpu)
 
 #endif /* #else #ifdef CONFIG_RCU_BOOST */
 
-#if !defined(CONFIG_RCU_FAST_NO_HZ)
+#if !defined(CONFIG_RCU_FAST_NO_HZ) || defined(CONFIG_PREEMPT_RT_FULL)
 
 /*
  * Check to see if any future RCU-related work will need to be done
@@ -1424,7 +1425,9 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 	*nextevt = KTIME_MAX;
 	return rcu_cpu_has_callbacks(NULL);
 }
+#endif /* !defined(CONFIG_RCU_FAST_NO_HZ) || defined(CONFIG_PREEMPT_RT_FULL) */
 
+#if !defined(CONFIG_RCU_FAST_NO_HZ)
 /*
  * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
  * after it.
@@ -1521,6 +1524,8 @@ static bool __maybe_unused rcu_try_advance_all_cbs(void)
 	return cbs_ready;
 }
 
+#ifndef CONFIG_PREEMPT_RT_FULL
+
 /*
  * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
  * to invoke.  If the CPU has callbacks, try to advance them.  Tell the
@@ -1563,6 +1568,7 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 	*nextevt = basemono + dj * TICK_NSEC;
 	return 0;
 }
+#endif /* #ifndef CONFIG_PREEMPT_RT_FULL */
 
 /*
  * Prepare a CPU for idle from an RCU perspective.  The first major task
-- 
2.17.1

