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Date: Sun, 13 Sep 2009 09:31:46 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Daniel Walker <dwalker@...o99.com>
Cc: linux-kernel@...r.kernel.org, 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
Subject: Re: [PATCH tip/core/rcu 2/4] Add debug checks to TREE_PREEMPT_RCU
for premature grace periods.
On Sun, Sep 13, 2009 at 09:23:02AM -0700, Daniel Walker wrote:
> On Sun, 2009-09-13 at 09:15 -0700, Paul E. McKenney wrote:
> > 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.
> >
>
> You've got a couple of whitespace issues in the WARN_ON_ONCE() lines..
> As found by checkpatch,
>
> ERROR: code indent should use tabs where possible
> #97: FILE: kernel/rcutree_plugin.h:89:
> +^I ^IWARN_ON_ONCE(cpu != smp_processor_id());$
>
> ERROR: code indent should use tabs where possible
> #109: FILE: kernel/rcutree_plugin.h:111:
> +^I ^IWARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);$
>
> Could you fix these up?
Good catch! Here is a corrected version.
Thanx, Paul
------------------------------------------------------------------------
>From f5807ddbd4fff957e6c2efdc874a740ff40f1c94 Mon Sep 17 00:00:00 2001
From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date: Tue, 8 Sep 2009 16:36:30 -0700
Subject: [PATCH tip/core/rcu 2/4] Add debug checks to TREE_PREEMPT_RCU for premature grace periods.
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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