[PATCH tip/core/rcu 19/21] rcu: Simplify and inline cpu_needs_another_gp()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Now that RCU no longer relies on failsafe checks, cpu_needs_another_gp()
can be greatly simplified.  This simplification eliminates the last
call to rcu_future_needs_gp() and to rcu_segcblist_future_gp_needed(),
both of which which can then be eliminated.  And then, because
cpu_needs_another_gp() is called only from __rcu_pending(), it can be
inlined and eliminated.

This commit carries out the simplification, inlining, and elimination
called out above.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/rcu_segcblist.c | 18 ------------------
 kernel/rcu/rcu_segcblist.h |  2 --
 kernel/rcu/tree.c          | 40 +++-------------------------------------
 3 files changed, 3 insertions(+), 57 deletions(-)

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 88cba7c2956c..5aff271adf1e 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -404,24 +404,6 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
 }
 
 /*
- * Scan the specified rcu_segcblist structure for callbacks that need
- * a grace period later than the one specified by "seq".  We don't look
- * at the RCU_DONE_TAIL or RCU_NEXT_TAIL segments because they don't
- * have a grace-period sequence number.
- */
-bool rcu_segcblist_future_gp_needed(struct rcu_segcblist *rsclp,
-				    unsigned long seq)
-{
-	int i;
-
-	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++)
-		if (rsclp->tails[i - 1] != rsclp->tails[i] &&
-		    ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
-			return true;
-	return false;
-}
-
-/*
  * Merge the source rcu_segcblist structure into the destination
  * rcu_segcblist structure, then initialize the source.  Any pending
  * callbacks from the source get to start over.  It is best to
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 581c12b63544..948470cef385 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -134,7 +134,5 @@ void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
 				   struct rcu_cblist *rclp);
 void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq);
 bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq);
-bool rcu_segcblist_future_gp_needed(struct rcu_segcblist *rsclp,
-				    unsigned long seq);
 void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
 			 struct rcu_segcblist *src_rsclp);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7776d709e060..020a0fe2dbee 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -709,42 +709,6 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
 }
 
 /*
- * Is there any need for future grace periods?
- * Interrupts must be disabled.  If the caller does not hold the root
- * rnp_node structure's ->lock, the results are advisory only.
- */
-static int rcu_future_needs_gp(struct rcu_state *rsp)
-{
-	struct rcu_node *rnp = rcu_get_root(rsp);
-
-	lockdep_assert_irqs_disabled();
-	return need_any_future_gp(rnp);
-}
-
-/*
- * Does the current CPU require a not-yet-started grace period?
- * The caller must have disabled interrupts to prevent races with
- * normal callback registry.
- */
-static bool
-cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
-{
-	lockdep_assert_irqs_disabled();
-	if (rcu_gp_in_progress(rsp))
-		return false;  /* No, a grace period is already in progress. */
-	if (rcu_future_needs_gp(rsp))
-		return true;  /* Yes, a no-CBs CPU needs one. */
-	if (!rcu_segcblist_is_enabled(&rdp->cblist))
-		return false;  /* No, this is a no-CBs (or offline) CPU. */
-	if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
-		return true;  /* Yes, CPU has newly registered callbacks. */
-	if (rcu_segcblist_future_gp_needed(&rdp->cblist,
-					   READ_ONCE(rsp->completed)))
-		return true;  /* Yes, CBs for future grace period. */
-	return false; /* No grace period needed. */
-}
-
-/*
  * Enter an RCU extended quiescent state, which can be either the
  * idle loop or adaptive-tickless usermode execution.
  *
@@ -3298,7 +3262,9 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
 		return 1;
 
 	/* Has RCU gone idle with this CPU needing another grace period? */
-	if (cpu_needs_another_gp(rsp, rdp))
+	if (!rcu_gp_in_progress(rsp) &&
+	    rcu_segcblist_is_enabled(&rdp->cblist) &&
+	    !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
 		return 1;
 
 	/* Has another RCU grace period completed?  */
-- 
2.5.2