rcu: Make ksoftirqd do RCU quiescent states

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 <[email protected]>

Link http://lkml.kernel.org/r/[email protected] open here

Signed-off-by Thomas Gleixner <[email protected]>

kernel/rcu/tree.c +13 -1

index 197088cdb56e..968579b86401 100644
@@ -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!!!");

kernel/rcu/tree_plugin.h +7 -1

index 429a2f144e19..bee9bffeb0ce 100644
@@ -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"

@@ -1407,7 +1408,7 @@ 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

@@ -1423,7 +1424,9 @@ 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.

@@ -1520,6 +1523,8 @@ 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

@@ -1562,6 +1567,7 @@ 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