[PATCH tip/core/rcu 07/19] rcu: Apply RCU-bh QSes to RCU-sched and RCU-preempt when safe

[Paul E. McKenney]

One necessary step towards consolidating the three flavors of RCU is to
make sure that the resulting consolidated "one flavor to rule them all"
correctly handles networking denial-of-service attacks.  One thing that
allows RCU-bh to do so is that __do_softirq() invokes rcu_bh_qs() every
so often, and so something similar has to happen for consolidated RCU.

This must be done carefully.  For example, if a preemption-disabled
region of code takes an interrupt which does softirq processing before
returning, consolidated RCU must ignore the resulting rcu_bh_qs()
invocations -- preemption is still disabled, and that means an RCU
reader for the consolidated flavor.

This commit therefore creates a new rcu_softirq_qs() that is called only
from the ksoftirqd task, thus avoiding the interrupted-a-preempted-region
problem.  This new rcu_softirq_qs() function invokes rcu_sched_qs(),
rcu_preempt_qs(), and rcu_preempt_deferred_qs().  The latter call handles
any deferred quiescent states.

Note that __do_softirq() still invokes rcu_bh_qs().  It will continue to
do so until a later stage of cleanup when the RCU-bh flavor is removed.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
[ paulmck: Fix !SMP issue located by kbuild test robot. ]
---
 include/linux/rcutiny.h  | 5 +++++
 include/linux/rcutree.h  | 1 +
 kernel/rcu/tree.c        | 7 +++++++
 kernel/rcu/tree.h        | 1 +
 kernel/rcu/tree_plugin.h | 5 +++++
 kernel/softirq.c         | 2 ++
 6 files changed, 21 insertions(+)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index f617ab19bb51..bcfbc40a7239 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -90,6 +90,11 @@ static inline void kfree_call_rcu(struct rcu_head *head,
 	call_rcu(head, func);
 }
 
+static inline void rcu_softirq_qs(void)
+{
+	rcu_sched_qs();
+}
+
 #define rcu_note_context_switch(preempt) \
 	do { \
 		rcu_sched_qs(); \
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 914655848ef6..664b580695d6 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -30,6 +30,7 @@
 #ifndef __LINUX_RCUTREE_H
 #define __LINUX_RCUTREE_H
 
+void rcu_softirq_qs(void);
 void rcu_note_context_switch(bool preempt);
 int rcu_needs_cpu(u64 basem, u64 *nextevt);
 void rcu_cpu_stall_reset(void);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d2b6ade692c9..9b6bd12133c4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -255,6 +255,13 @@ void rcu_bh_qs(void)
 	}
 }
 
+void rcu_softirq_qs(void)
+{
+	rcu_sched_qs();
+	rcu_preempt_qs();
+	rcu_preempt_deferred_qs(current);
+}
+
 /*
  * Steal a bit from the bottom of ->dynticks for idle entry/exit
  * control.  Initially this is for TLB flushing.
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 025bd2e5592b..e02c882861eb 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -433,6 +433,7 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);
 
 /* Forward declarations for rcutree_plugin.h */
 static void rcu_bootup_announce(void);
+static void rcu_preempt_qs(void);
 static void rcu_preempt_note_context_switch(bool preempt);
 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
 #ifdef CONFIG_HOTPLUG_CPU
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 527a52792dce..c686bf63bba5 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -974,6 +974,11 @@ static void __init rcu_bootup_announce(void)
 	rcu_bootup_announce_oddness();
 }
 
+/* Because preemptible RCU does not exist, we can ignore its QSes. */
+static void rcu_preempt_qs(void)
+{
+}
+
 /*
  * Because preemptible RCU does not exist, we never have to check for
  * CPUs being in quiescent states.
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 6f584861d329..ebd69694144a 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -302,6 +302,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 	}
 
 	rcu_bh_qs();
+	if (__this_cpu_read(ksoftirqd) == current)
+		rcu_softirq_qs();
 	local_irq_disable();
 
 	pending = local_softirq_pending();
-- 
2.17.1