linux-kernel - [PATCH tip/core/rcu 2/4] Add debug checks to TREE_PREEMPT

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Message-Id: <12528585111986-git-send-email->
Date:	Sun, 13 Sep 2009 09:15:09 -0700
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,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 2/4] Add debug checks to TREE_PREEMPT_RCU for premature grace periods.

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

Check to make sure that there are no blocked tasks for the previous
grace period while initializing for the next grace period, verify
that rcu_preempt_qs() is given the correct CPU number and is never
called for an offline CPU.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutree.c        |    2 ++
 kernel/rcutree_plugin.h |   25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 0 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index bca0aba..3a01405 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -627,6 +627,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	/* Special-case the common single-level case. */
 	if (NUM_RCU_NODES == 1) {
 		rnp->qsmask = rnp->qsmaskinit;
+		rcu_preempt_check_blocked_tasks(rnp);
 		rnp->gpnum = rsp->gpnum;
 		rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
 		spin_unlock_irqrestore(&rnp->lock, flags);
@@ -660,6 +661,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) {
 		spin_lock(&rnp_cur->lock);	/* irqs already disabled. */
 		rnp_cur->qsmask = rnp_cur->qsmaskinit;
+		rcu_preempt_check_blocked_tasks(rnp);
 		rnp->gpnum = rsp->gpnum;
 		spin_unlock(&rnp_cur->lock);	/* irqs already disabled. */
 	}
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 4778936..51413cb 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -86,6 +86,7 @@ static void rcu_preempt_qs(int cpu)
 
 	if (t->rcu_read_lock_nesting &&
 	    (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
+	    	WARN_ON_ONCE(cpu != smp_processor_id());
 
 		/* Possibly blocking in an RCU read-side critical section. */
 		rdp = rcu_preempt_state.rda[cpu];
@@ -103,7 +104,11 @@ static void rcu_preempt_qs(int cpu)
 		 * state for the current grace period), then as long
 		 * as that task remains queued, the current grace period
 		 * cannot end.
+		 *
+		 * But first, note that the current CPU must still be
+		 * on line!
 		 */
+	    	WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
 		phase = !(rnp->qsmask & rdp->grpmask) ^ (rnp->gpnum & 0x1);
 		list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
 		smp_mb();  /* Ensure later ctxt swtch seen after above. */
@@ -259,6 +264,18 @@ static void rcu_print_task_stall(struct rcu_node *rnp)
 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
 /*
+ * Check that the list of blocked tasks for the newly completed grace
+ * period is in fact empty.  It is a serious bug to complete a grace
+ * period that still has RCU readers blocked!  This function must be
+ * invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
+ * must be held by the caller.
+ */
+static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
+{
+	WARN_ON_ONCE(!list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]));
+}
+
+/*
  * Check for preempted RCU readers for the specified rcu_node structure.
  * If the caller needs a reliable answer, it must hold the rcu_node's
  * >lock.
@@ -451,6 +468,14 @@ static void rcu_print_task_stall(struct rcu_node *rnp)
 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
 /*
+ * Because there is no preemptable RCU, there can be no readers blocked,
+ * so there is no need to check for blocked tasks.
+ */
+static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
+{
+}
+
+/*
  * Because preemptable RCU does not exist, there are never any preempted
  * RCU readers.
  */
-- 
1.5.2.5

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