Re: INFO: possible circular locking dependency detected

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 2011-07-15 at 15:07 +0200, Peter Zijlstra wrote:

> OK, so the latter case cannot happen (rcu_preempt_check_callbacks only
> sets NEED_QS when rcu_read_lock_nesting), we need two interrupts for
> this to happen.
> 
> rcu_read_lock()
> 
>  <IRQ>
>    |= RCU_READ_UNLOCK_NEED_QS
> 
> rcu_read_unlock()
>   __rcu_read_unlock()
>    --rcu_read_lock_nesting;
>      <IRQ>
> 	ttwu()
>           rcu_read_lock()
> 	  rcu_read_unlock()
> 	    rcu_read_unlock_special()
> 	      *BANG*
>    rcu_read_unlock_special()
> 

What about this patch? Not even compiled tested.

-- Steve

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 14dc7dd..e3545fa 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -284,18 +284,17 @@ static struct list_head *rcu_next_node_entry(struct task_struct *t,
  * notify RCU core processing or task having blocked during the RCU
  * read-side critical section.
  */
-static void rcu_read_unlock_special(struct task_struct *t)
+static int rcu_read_unlock_special(struct task_struct *t, int special)
 {
 	int empty;
 	int empty_exp;
 	unsigned long flags;
 	struct list_head *np;
 	struct rcu_node *rnp;
-	int special;
 
 	/* NMI handlers cannot block and cannot safely manipulate state. */
 	if (in_nmi())
-		return;
+		return special;
 
 	local_irq_save(flags);
 
@@ -303,7 +302,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	 * If RCU core is waiting for this CPU to exit critical section,
 	 * let it know that we have done so.
 	 */
-	special = t->rcu_read_unlock_special;
 	if (special & RCU_READ_UNLOCK_NEED_QS) {
 		rcu_preempt_qs(smp_processor_id());
 	}
@@ -311,7 +309,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	/* Hardware IRQ handlers cannot block. */
 	if (in_irq()) {
 		local_irq_restore(flags);
-		return;
+		return special;
 	}
 
 	/* Clean up if blocked during RCU read-side critical section. */
@@ -373,6 +371,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	} else {
 		local_irq_restore(flags);
 	}
+	return special;
 }
 
 /*
@@ -385,13 +384,21 @@ static void rcu_read_unlock_special(struct task_struct *t)
 void __rcu_read_unlock(void)
 {
 	struct task_struct *t = current;
+	int special;
 
+	special = ACCESS_ONCE(t->rcu_read_unlock_special);
+	/*
+	 * Clear special here to prevent interrupts from seeing it
+	 * enabled after decrementing lock_nesting and calling
+	 * rcu_read_unlock_special().
+	 */
+	t->rcu_read_unlock_special = 0;
 	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
 	--t->rcu_read_lock_nesting;
 	barrier();  /* decrement before load of ->rcu_read_unlock_special */
-	if (t->rcu_read_lock_nesting == 0 &&
-	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
-		rcu_read_unlock_special(t);
+	if (t->rcu_read_lock_nesting == 0 && special)
+		special = rcu_read_unlock_special(t, special);
+	t->rcu_read_unlock_special = special;
 #ifdef CONFIG_PROVE_LOCKING
 	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
 #endif /* #ifdef CONFIG_PROVE_LOCKING */