There are several critical sections that require synchronization with
synchronize_sched(). Usually these are done by disabling preemption and
the synchronize_sched() just waits for the kernel to schedule on each
of the CPUs.

The rcu_read_lock_sched() is the preferred API to use, but some areas
still use preempt_disable() and local_irq_*() to prevent preemption
from happening. But our main concern is with those users of
rcu_read_lock_sched(), where they may also call spin_locks() that turn
into a mutex for PREEMPT_RT. For these cases, we need to allow
rcu_read_lock_sched() to schedule out.

To allow rcu_read_lock_sched() sections to preempt when PREEMPT_RT is enabled,
instead of disabling preemption, it will grab a local_lock(). Then the
synchronize_sched() will grab all CPUs local_locks() and release them.
After that, it still does the normal synchronize_sched() as there may be
places that still disable preemption or irqs that it needs to
synchronize with. By grabbing all the locks and releasing them, it will
properly synchronize with those that use the locks instead of disabling
preemption or interrupts.

Note: The rcu_read_lock_sched_notrace() version still only disables
preemption, because they are used for lockdep and tracing, which require
real preemption disabling and not mutexes.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Index: linux-rt.git/include/linux/rcupdate.h
===================================================================
--- linux-rt.git.orig/include/linux/rcupdate.h
+++ linux-rt.git/include/linux/rcupdate.h
@@ -36,6 +36,7 @@
 #include <linux/types.h>
 #include <linux/cache.h>
 #include <linux/spinlock.h>
+#include <linux/locallock.h>
 #include <linux/threads.h>
 #include <linux/cpumask.h>
 #include <linux/seqlock.h>
@@ -870,6 +871,28 @@ static inline void rcu_read_unlock_bh(vo
 	local_bh_enable();
 }
 
+/* asm-offsets.c gets confused with local_lock here */
+#if defined(CONFIG_PREEMPT_RT_FULL)
+DECLARE_LOCAL_IRQ_LOCK(rcu_sched_lock);
+static inline void rcu_read_lock_sched_disable(void)
+{
+	local_lock(rcu_sched_lock);
+}
+static inline void rcu_read_lock_sched_enable(void)
+{
+	local_unlock(rcu_sched_lock);
+}
+#else
+static inline void rcu_read_lock_sched_disable(void)
+{
+	preempt_disable();
+}
+static inline void rcu_read_lock_sched_enable(void)
+{
+	preempt_enable();
+}
+#endif
+
 /**
  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
  *
@@ -885,7 +908,7 @@ static inline void rcu_read_unlock_bh(vo
  */
 static inline void rcu_read_lock_sched(void)
 {
-	preempt_disable();
+	rcu_read_lock_sched_disable();
 	__acquire(RCU_SCHED);
 	rcu_lock_acquire(&rcu_sched_lock_map);
 	rcu_lockdep_assert(!rcu_is_cpu_idle(),
@@ -910,7 +933,7 @@ static inline void rcu_read_unlock_sched
 			   "rcu_read_unlock_sched() used illegally while idle");
 	rcu_lock_release(&rcu_sched_lock_map);
 	__release(RCU_SCHED);
-	preempt_enable();
+	rcu_read_lock_sched_enable();
 }
 
 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
Index: linux-rt.git/kernel/rcutree.c
===================================================================
--- linux-rt.git.orig/kernel/rcutree.c
+++ linux-rt.git/kernel/rcutree.c
@@ -2491,6 +2491,31 @@ static inline int rcu_blocking_is_gp(voi
 	return ret;
 }
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+DEFINE_LOCAL_IRQ_LOCK(rcu_sched_lock);
+/*
+ * Real-time allows for synchronize sched to sleep but not migrate.
+ * This is done via the local locks. When calling synchronize_sched(),
+ * all the local locks need to be taken and released. This will ensure
+ * that all users of rcu_read_lock_sched() will be out of their critical
+ * sections at the completion of this function. synchronize_sched() will
+ * still perform the normal RCU sched operations to synchronize with
+ * locations that use disabling preemption or interrupts.
+ */
+static void rcu_synchronize_sched_rt(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		spin_lock(&per_cpu(rcu_sched_lock, cpu).lock);
+		spin_unlock(&per_cpu(rcu_sched_lock, cpu).lock);
+	}
+}
+#else
+static inline void rcu_synchronize_sched_rt(void)
+{
+}
+#endif
 /**
  * synchronize_sched - wait until an rcu-sched grace period has elapsed.
  *
@@ -2538,6 +2563,9 @@ void synchronize_sched(void)
 			   !lock_is_held(&rcu_lock_map) &&
 			   !lock_is_held(&rcu_sched_lock_map),
 			   "Illegal synchronize_sched() in RCU-sched read-side critical section");
+
+	rcu_synchronize_sched_rt();
+
 	if (rcu_blocking_is_gp())
 		return;
 	if (rcu_expedited)

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