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SubjectRe: INFO: possible circular locking dependency detected
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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 */



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