3 files changed, 70 insertions, 4 deletions

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e8cb6e3b52a7..425c659d54e5 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -27,6 +27,16 @@
 
 #include <linux/cache.h>
 
+static inline unsigned long get_state_synchronize_rcu(void)
+{
+        return 0;
+}
+
+static inline void cond_synchronize_rcu(unsigned long oldstate)
+{
+        might_sleep();
+}
+
 static inline void rcu_barrier_bh(void)
 {
         wait_rcu_gp(call_rcu_bh);
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index e9c63884df0a..a59ca05fd4e3 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -76,6 +76,8 @@ static inline void synchronize_rcu_bh_expedited(void)
 void rcu_barrier(void);
 void rcu_barrier_bh(void);
 void rcu_barrier_sched(void);
+unsigned long get_state_synchronize_rcu(void);
+void cond_synchronize_rcu(unsigned long oldstate);
 
 extern unsigned long rcutorture_testseq;
 extern unsigned long rcutorture_vernum;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 351faba48b91..0c47e300210a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1421,13 +1421,14 @@ static int rcu_gp_init(struct rcu_state *rsp)
 
         /* Advance to a new grace period and initialize state. */
         record_gp_stall_check_time(rsp);
-        smp_wmb(); /* Record GP times before starting GP. */
-        rsp->gpnum++;
+        /* Record GP times before starting GP, hence smp_store_release(). */
+        smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
         trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
         raw_spin_unlock_irq(&rnp->lock);
 
         /* Exclude any concurrent CPU-hotplug operations. */
         mutex_lock(&rsp->onoff_mutex);
+        smp_mb__after_unlock_lock(); /* ->gpnum increment before GP! */
 
         /*
          * Set the quiescent-state-needed bits in all the rcu_node
@@ -1555,10 +1556,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
         }
         rnp = rcu_get_root(rsp);
         raw_spin_lock_irq(&rnp->lock);
-        smp_mb__after_unlock_lock();
+        smp_mb__after_unlock_lock(); /* Order GP before ->completed update. */
         rcu_nocb_gp_set(rnp, nocb);
 
-        rsp->completed = rsp->gpnum; /* Declare grace period done. */
+        /* Declare grace period done. */
+        ACCESS_ONCE(rsp->completed) = rsp->gpnum;
         trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
         rsp->fqs_state = RCU_GP_IDLE;
         rdp = this_cpu_ptr(rsp->rda);
@@ -2637,6 +2639,58 @@ void synchronize_rcu_bh(void)
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 
+/**
+ * get_state_synchronize_rcu - Snapshot current RCU state
+ *
+ * Returns a cookie that is used by a later call to cond_synchronize_rcu()
+ * to determine whether or not a full grace period has elapsed in the
+ * meantime.
+ */
+unsigned long get_state_synchronize_rcu(void)
+{
+        /*
+         * Any prior manipulation of RCU-protected data must happen
+         * before the load from ->gpnum.
+         */
+        smp_mb();  /* ^^^ */
+
+        /*
+         * Make sure this load happens before the purportedly
+         * time-consuming work between get_state_synchronize_rcu()
+         * and cond_synchronize_rcu().
+         */
+        return smp_load_acquire(&rcu_state->gpnum);
+}
+EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
+
+/**
+ * cond_synchronize_rcu - Conditionally wait for an RCU grace period
+ *
+ * @oldstate: return value from earlier call to get_state_synchronize_rcu()
+ *
+ * If a full RCU grace period has elapsed since the earlier call to
+ * get_state_synchronize_rcu(), just return.  Otherwise, invoke
+ * synchronize_rcu() to wait for a full grace period.
+ *
+ * Yes, this function does not take counter wrap into account.  But
+ * counter wrap is harmless.  If the counter wraps, we have waited for
+ * more than 2 billion grace periods (and way more on a 64-bit system!),
+ * so waiting for one additional grace period should be just fine.
+ */
+void cond_synchronize_rcu(unsigned long oldstate)
+{
+        unsigned long newstate;
+
+        /*
+         * Ensure that this load happens before any RCU-destructive
+         * actions the caller might carry out after we return.
+         */
+        newstate = smp_load_acquire(&rcu_state->completed);
+        if (ULONG_CMP_GE(oldstate, newstate))
+                synchronize_rcu();
+}
+EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
+
 static int synchronize_sched_expedited_cpu_stop(void *data)
 {
         /*