linux-kernel - [PATCH RFC tip/core/rcu 08/18] rcu: remove leg of force_quiescent

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Message-Id: <1260918161349-git-send-email->
Date:	Tue, 15 Dec 2009 15:02:31 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 08/18] rcu: remove leg of force_quiescent_state() switch statement

From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Proposed for 2.6.34, not for inclusion.

The comparisons of rsp->gpnum nad rsp->completed in rcu_process_dyntick()
and force_quiescent_state() can be replaced by the much more clear
rcu_gp_in_progress() predicate function.  After doing this, it becomes
clear that the RCU_SAVE_COMPLETED leg of the force_quiescent_state()
function's switch statement is almost completely a no-op.  A small change
to the RCU_SAVE_DYNTICK leg renders it a complete no-op, after which it
can be removed.  Doing so also eliminates the forcenow local variable
from force_quiescent_state().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutree.c |   22 +++++-----------------
 kernel/rcutree.h |    5 ++---
 2 files changed, 7 insertions(+), 20 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e497119..6268f37 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1144,6 +1144,7 @@ void rcu_check_callbacks(int cpu, int user)
 /*
  * Scan the leaf rcu_node structures, processing dyntick state for any that
  * have not yet encountered a quiescent state, using the function specified.
+ * The caller must have suppressed start of new grace periods.
  */
 static void rcu_process_dyntick(struct rcu_state *rsp,
 				int (*f)(struct rcu_data *))
@@ -1157,7 +1158,7 @@ static void rcu_process_dyntick(struct rcu_state *rsp,
 	rcu_for_each_leaf_node(rsp, rnp) {
 		mask = 0;
 		spin_lock_irqsave(&rnp->lock, flags);
-		if (rnp->completed != rsp->gpnum - 1) {
+		if (!rcu_gp_in_progress(rsp)) {
 			spin_unlock_irqrestore(&rnp->lock, flags);
 			return;
 		}
@@ -1171,7 +1172,7 @@ static void rcu_process_dyntick(struct rcu_state *rsp,
 			if ((rnp->qsmask & bit) != 0 && f(rsp->rda[cpu]))
 				mask |= bit;
 		}
-		if (mask != 0 && rnp->completed == rsp->gpnum - 1) {
+		if (mask != 0 && rcu_gp_in_progress(rsp)) {
 
 			/* rcu_report_qs_rnp() releases rnp->lock. */
 			rcu_report_qs_rnp(mask, rsp, rnp, flags);
@@ -1189,7 +1190,6 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
 {
 	unsigned long flags;
 	struct rcu_node *rnp = rcu_get_root(rsp);
-	u8 forcenow;
 
 	if (!rcu_gp_in_progress(rsp))
 		return;  /* No grace period in progress, nothing to force. */
@@ -1224,21 +1224,9 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
 		/* Record dyntick-idle state. */
 		rcu_process_dyntick(rsp, dyntick_save_progress_counter);
 		spin_lock(&rnp->lock);  /* irqs already disabled */
-		if (!rcu_gp_in_progress(rsp))
-			break;
-		/* fall into next case. */
-
-	case RCU_SAVE_COMPLETED:
-
-		/* Update state, record completion counter. */
-		forcenow = 0;
-		if (rsp->gpnum - 1 == rsp->completed) {
-			forcenow = rsp->signaled == RCU_SAVE_COMPLETED;
+		if (rcu_gp_in_progress(rsp))
 			rsp->signaled = RCU_FORCE_QS;
-		}
-		if (!forcenow)
-			break;
-		/* fall into next case. */
+		break;
 
 	case RCU_FORCE_QS:
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 5348561..edb6fae 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -237,12 +237,11 @@ struct rcu_data {
 #define RCU_GP_IDLE		0	/* No grace period in progress. */
 #define RCU_GP_INIT		1	/* Grace period being initialized. */
 #define RCU_SAVE_DYNTICK	2	/* Need to scan dyntick state. */
-#define RCU_SAVE_COMPLETED	3	/* Need to save rsp->completed. */
-#define RCU_FORCE_QS		4	/* Need to force quiescent state. */
+#define RCU_FORCE_QS		3	/* Need to force quiescent state. */
 #ifdef CONFIG_NO_HZ
 #define RCU_SIGNAL_INIT		RCU_SAVE_DYNTICK
 #else /* #ifdef CONFIG_NO_HZ */
-#define RCU_SIGNAL_INIT		RCU_SAVE_COMPLETED
+#define RCU_SIGNAL_INIT		RCU_FORCE_QS
 #endif /* #else #ifdef CONFIG_NO_HZ */
 
 #define RCU_JIFFIES_TILL_FORCE_QS	 3	/* for rsp->jiffies_force_qs */
-- 
1.5.2.5

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