linux-kernel - [PATCH 1/2] rcu: Show the real fqs

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Date:	Fri,  4 Sep 2015 14:11:29 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Jiri Kosina <jkosina@...e.com>, linux-kernel@...r.kernel.org,
	Petr Mladek <pmladek@...e.com>
Subject: [PATCH 1/2] rcu: Show the real fqs_state

The value of "fqs_state" in struct rcu_state is always RCU_GP_IDLE.

The real state is stored in a local variable in rcu_gp_kthread().
It is modified by rcu_gp_fqs() via parameter and return value.
But the actual value is never stored to rsp->fqs_state.

The result is that print_one_rcu_state() does not show the real
state.

This code has been added 3 years ago by the commit 4cdfc175c25c89ee
("rcu: Move quiescent-state forcing into kthread"). I guess that it
was an overlook or optimization.

Anyway, the value seems to be manipulated only by the thread, except
for shoving the status. I do not see any risk in updating it directly
in the struct.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 kernel/rcu/tree.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9f75f25cc5d9..54af8d5f9f7b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1927,16 +1927,15 @@ static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
 /*
  * Do one round of quiescent-state forcing.
  */
-static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
+static void rcu_gp_fqs(struct rcu_state *rsp)
 {
-	int fqs_state = fqs_state_in;
 	bool isidle = false;
 	unsigned long maxj;
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	WRITE_ONCE(rsp->gp_activity, jiffies);
 	rsp->n_force_qs++;
-	if (fqs_state == RCU_SAVE_DYNTICK) {
+	if (rsp->fqs_state == RCU_SAVE_DYNTICK) {
 		/* Collect dyntick-idle snapshots. */
 		if (is_sysidle_rcu_state(rsp)) {
 			isidle = true;
@@ -1945,7 +1944,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 		force_qs_rnp(rsp, dyntick_save_progress_counter,
 			     &isidle, &maxj);
 		rcu_sysidle_report_gp(rsp, isidle, maxj);
-		fqs_state = RCU_FORCE_QS;
+		rsp->fqs_state = RCU_FORCE_QS;
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
 		isidle = true;
@@ -1959,7 +1958,6 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 			   READ_ONCE(rsp->gp_flags) & ~RCU_GP_FLAG_FQS);
 		raw_spin_unlock_irq(&rnp->lock);
 	}
-	return fqs_state;
 }
 
 /*
@@ -2041,7 +2039,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
  */
 static int __noreturn rcu_gp_kthread(void *arg)
 {
-	int fqs_state;
 	int gf;
 	unsigned long j;
 	int ret;
@@ -2073,7 +2070,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 		}
 
 		/* Handle quiescent-state forcing. */
-		fqs_state = RCU_SAVE_DYNTICK;
+		rsp->fqs_state = RCU_SAVE_DYNTICK;
 		j = jiffies_till_first_fqs;
 		if (j > HZ) {
 			j = HZ;
@@ -2101,7 +2098,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 				trace_rcu_grace_period(rsp->name,
 						       READ_ONCE(rsp->gpnum),
 						       TPS("fqsstart"));
-				fqs_state = rcu_gp_fqs(rsp, fqs_state);
+				rcu_gp_fqs(rsp);
 				trace_rcu_grace_period(rsp->name,
 						       READ_ONCE(rsp->gpnum),
 						       TPS("fqsend"));
-- 
1.8.5.6

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