rcu: 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@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
LKML-Reference: <12528585111986-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index da301e2..e9a4ae9 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -632,6 +632,7 @@ 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);
@@ -665,6 +666,7 @@ 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..b8e4b03 100644 (file)
--- 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. */
@@ -258,6 +263,18 @@ 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
@@ -450,6 +467,14 @@ 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.