5 files changed, 45 insertions, 3 deletions

diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index e83b26464061..b4e2fa52d8bc 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -20,7 +20,7 @@
 #include <linux/capability.h>
 #include <linux/compiler.h>
 
-#include <linux/rcupdate.h>     /* rcu_expedited */
+#include <linux/rcupdate.h>     /* rcu_expedited and rcu_normal */
 
 #define KERNEL_ATTR_RO(_name) \
 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
@@ -148,7 +148,7 @@ int rcu_expedited;
 static ssize_t rcu_expedited_show(struct kobject *kobj,
                                   struct kobj_attribute *attr, char *buf)
 {
-        return sprintf(buf, "%d\n", rcu_expedited);
+        return sprintf(buf, "%d\n", READ_ONCE(rcu_expedited));
 }
 static ssize_t rcu_expedited_store(struct kobject *kobj,
                                    struct kobj_attribute *attr,
@@ -161,6 +161,23 @@ static ssize_t rcu_expedited_store(struct kobject *kobj,
 }
 KERNEL_ATTR_RW(rcu_expedited);
 
+int rcu_normal;
+static ssize_t rcu_normal_show(struct kobject *kobj,
+                               struct kobj_attribute *attr, char *buf)
+{
+        return sprintf(buf, "%d\n", READ_ONCE(rcu_normal));
+}
+static ssize_t rcu_normal_store(struct kobject *kobj,
+                                struct kobj_attribute *attr,
+                                const char *buf, size_t count)
+{
+        if (kstrtoint(buf, 0, &rcu_normal))
+                return -EINVAL;
+
+        return count;
+}
+KERNEL_ATTR_RW(rcu_normal);
+
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
  */
@@ -203,6 +220,7 @@ static struct attribute * kernel_attrs[] = {
         &vmcoreinfo_attr.attr,
 #endif
         &rcu_expedited_attr.attr,
+        &rcu_normal_attr.attr,
         NULL
 };
 
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index a63a1ea5a41b..9b9cdd549caa 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -489,7 +489,7 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
  */
 void synchronize_srcu(struct srcu_struct *sp)
 {
-        __synchronize_srcu(sp, rcu_gp_is_expedited()
+        __synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
                            ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
                            : SYNCHRONIZE_SRCU_TRYCOUNT);
 }
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6a652d1f3d7f..489992997c06 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3841,6 +3841,12 @@ void synchronize_sched_expedited(void)
         if (rcu_blocking_is_gp())
                 return;
 
+        /* If expedited grace periods are prohibited, fall back to normal. */
+        if (rcu_gp_is_normal()) {
+                wait_rcu_gp(call_rcu_sched);
+                return;
+        }
+
         /* Take a snapshot of the sequence number.  */
         s = rcu_exp_gp_seq_snap(rsp);
 
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 57ba873d2f18..d45df3781551 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -746,6 +746,12 @@ void synchronize_rcu_expedited(void)
         struct rcu_state *rsp = rcu_state_p;
         unsigned long s;
 
+        /* If expedited grace periods are prohibited, fall back to normal. */
+        if (rcu_gp_is_normal()) {
+                wait_rcu_gp(call_rcu);
+                return;
+        }
+
         s = rcu_exp_gp_seq_snap(rsp);
 
         rnp_unlock = exp_funnel_lock(rsp, s);
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5f748c5a40f0..8fccda3a794d 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -61,6 +61,7 @@ MODULE_ALIAS("rcupdate");
 #define MODULE_PARAM_PREFIX "rcupdate."
 
 module_param(rcu_expedited, int, 0);
+module_param(rcu_normal, int, 0);
 
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
@@ -113,6 +114,17 @@ EXPORT_SYMBOL(rcu_read_lock_sched_held);
 
 #ifndef CONFIG_TINY_RCU
 
+/*
+ * Should expedited grace-period primitives always fall back to their
+ * non-expedited counterparts?  Intended for use within RCU.  Note
+ * that if the user specifies both rcu_expedited and rcu_normal, then
+ * rcu_normal wins.
+ */
+bool rcu_gp_is_normal(void)
+{
+        return READ_ONCE(rcu_normal);
+}
+
 static atomic_t rcu_expedited_nesting =
         ATOMIC_INIT(IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT) ? 1 : 0);