Re: [PATCH 1/2] rcu: Show the real fqs_state

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

On Fri, Sep 04, 2015 at 02:11:29PM +0200, Petr Mladek wrote:
> 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@suse.com>

Good catch, but how about the following fix instead?

							Thanx, Paul

------------------------------------------------------------------------

    rcu: Finish folding ->fqs_state into ->gp_state
    
    Commit commit 4cdfc175c25c89ee ("rcu: Move quiescent-state forcing
    into kthread") started the process of folding the old ->fqs_state
    into ->gp_state, but did not complete it.  This situation does not
    cause any malfunction, but can result in extremely confusing trace
    output.  This commit completes this task of eliminating ->fqs_state
    in favor of ->gp_state.
    
    Reported-by: Petr Mladek <pmladek@suse.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 69ab7ce2cf7b..04234936d897 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -97,7 +97,7 @@ struct rcu_state sname##_state = { \
 	.level = { &sname##_state.node[0] }, \
 	.rda = &sname##_data, \
 	.call = cr, \
-	.fqs_state = RCU_GP_IDLE, \
+	.gp_state = RCU_GP_IDLE, \
 	.gpnum = 0UL - 300UL, \
 	.completed = 0UL - 300UL, \
 	.orphan_lock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.orphan_lock), \
@@ -1949,16 +1949,15 @@ 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->gp_state == RCU_SAVE_DYNTICK) {
 		/* Collect dyntick-idle snapshots. */
 		if (is_sysidle_rcu_state(rsp)) {
 			isidle = true;
@@ -1967,7 +1966,7 @@ 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->gp_state = RCU_FORCE_QS;
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
 		isidle = true;
@@ -1981,7 +1980,6 @@ 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;
 }
 
 /*
@@ -2045,7 +2043,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	/* Declare grace period done. */
 	WRITE_ONCE(rsp->completed, rsp->gpnum);
 	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
-	rsp->fqs_state = RCU_GP_IDLE;
+	rsp->gp_state = RCU_GP_IDLE;
 	rdp = this_cpu_ptr(rsp->rda);
 	/* Advance CBs to reduce false positives below. */
 	needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
@@ -2063,7 +2061,6 @@ 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;
@@ -2095,7 +2092,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 		}
 
 		/* Handle quiescent-state forcing. */
-		fqs_state = RCU_SAVE_DYNTICK;
+		rsp->gp_state = RCU_SAVE_DYNTICK;
 		j = jiffies_till_first_fqs;
 		if (j > HZ) {
 			j = HZ;
@@ -2123,7 +2120,7 @@ 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"));
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d5f58e717c8b..9faad70a8246 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -417,12 +417,11 @@ struct rcu_data {
 	struct rcu_state *rsp;
 };
 
-/* Values for fqs_state field in struct rcu_state. */
+/* Values for gp_state field in struct rcu_state. */
 #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_FORCE_QS		3	/* Need to force quiescent state. */
-#define RCU_SIGNAL_INIT		RCU_SAVE_DYNTICK
 
 /* Values for nocb_defer_wakeup field in struct rcu_data. */
 #define RCU_NOGP_WAKE_NOT	0
@@ -474,9 +473,8 @@ struct rcu_state {
 
 	/* The following fields are guarded by the root rcu_node's lock. */
 
-	u8	fqs_state ____cacheline_internodealigned_in_smp;
-						/* Force QS state. */
-	u8	boost;				/* Subject to priority boost. */
+	u8	boost ____cacheline_internodealigned_in_smp;
+						/* Subject to priority boost. */
 	unsigned long gpnum;			/* Current gp number. */
 	unsigned long completed;		/* # of last completed gp. */
 	struct task_struct *gp_kthread;		/* Task for grace periods. */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index 999c3672f990..ef7093cc9b5c 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -268,7 +268,7 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
 	gpnum = rsp->gpnum;
 	seq_printf(m, "c=%ld g=%ld s=%d jfq=%ld j=%x ",
 		   ulong2long(rsp->completed), ulong2long(gpnum),
-		   rsp->fqs_state,
+		   rsp->gp_state,
 		   (long)(rsp->jiffies_force_qs - jiffies),
 		   (int)(jiffies & 0xffff));
 	seq_printf(m, "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",