rcu: Convert conditional grace-period primitives to ->gp_seq

This commit converts get_state_synchronize_rcu(), cond_synchronize_rcu(),
get_state_synchronize_sched(), and cond_synchronize_sched() from ->gpnum
and ->completed to ->gp_seq.  Note that this also introduces a full
memory barrier in the already-done paths off cond_synchronize_rcu() and
cond_synchronize_sched(), as work with LKMM indicates that the earlier
smp_load_acquire() were insufficiently strong in some situations where
these two functions were called just as the grace period ended.  In such
cases, these two functions would not gain the benefit of memory ordering
at the end of the grace period.

Please note that the performance impact is negligible, as you shouldn't
be using either function anywhere near a fastpath in any case.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3183,16 +3183,10 @@ unsigned long get_state_synchronize_rcu(void)
 {
 	/*
 	 * Any prior manipulation of RCU-protected data must happen
-	 * before the load from ->gpnum.
+	 * before the load from ->gp_seq.
 	 */
 	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_p->gpnum);
+	return rcu_seq_snap(&rcu_state_p->gp_seq);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
 
@@ -3212,15 +3206,10 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
  */
 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_p->completed);
-	if (ULONG_CMP_GE(oldstate, newstate))
+	if (!rcu_seq_done(&rcu_state_p->gp_seq, oldstate))
 		synchronize_rcu();
+	else
+		smp_mb(); /* Ensure GP ends before subsequent accesses. */
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
 
@@ -3235,16 +3224,10 @@ unsigned long get_state_synchronize_sched(void)
 {
 	/*
 	 * Any prior manipulation of RCU-protected data must happen
-	 * before the load from ->gpnum.
+	 * before the load from ->gp_seq.
 	 */
 	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);
+	return rcu_seq_snap(&rcu_sched_state.gp_seq);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_sched);
 
@@ -3264,15 +3247,10 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_sched);
  */
 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))
+	if (!rcu_seq_done(&rcu_sched_state.gp_seq, oldstate))
 		synchronize_sched();
+	else
+		smp_mb(); /* Ensure GP ends before subsequent accesses. */
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_sched);