rcu: Add RCU-sched flavors of get-state and cond-sync

The get_state_synchronize_rcu() and cond_synchronize_rcu() functions
allow polling for grace-period completion, with an actual wait for a
grace period occurring only when cond_synchronize_rcu() is called too
soon after the corresponding get_state_synchronize_rcu().  However,
these functions work only for vanilla RCU.  This commit adds the
get_state_synchronize_sched() and cond_synchronize_sched(), which provide
the same capability for RCU-sched.

Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 52 insertions, 0 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8b5dd8ba9495..9629298eea24 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3253,6 +3253,58 @@ void cond_synchronize_rcu(unsigned long oldstate)
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
 
+/**
+ * get_state_synchronize_sched - Snapshot current RCU-sched state
+ *
+ * Returns a cookie that is used by a later call to cond_synchronize_sched()
+ * to determine whether or not a full grace period has elapsed in the
+ * meantime.
+ */
+unsigned long get_state_synchronize_sched(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_sched()
+         * and cond_synchronize_sched().
+         */
+        return smp_load_acquire(&rcu_sched_state.gpnum);
+}
+EXPORT_SYMBOL_GPL(get_state_synchronize_sched);
+
+/**
+ * cond_synchronize_sched - Conditionally wait for an RCU-sched grace period
+ *
+ * @oldstate: return value from earlier call to get_state_synchronize_sched()
+ *
+ * If a full RCU-sched grace period has elapsed since the earlier call to
+ * get_state_synchronize_sched(), just return.  Otherwise, invoke
+ * synchronize_sched() 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_sched(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_sched_state.completed);
+        if (ULONG_CMP_GE(oldstate, newstate))
+                synchronize_sched();
+}
+EXPORT_SYMBOL_GPL(cond_synchronize_sched);
+
 static int synchronize_sched_expedited_cpu_stop(void *data)
 {
         /*