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SubjectRe: INFO: possible circular locking dependency detected
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On Fri, 2011-07-15 at 10:24 -0700, Paul E. McKenney wrote:

> But the rcu_read_unlock() called from within the irq handler would
> take a second snapshot of ->special. It could then enter
> rcu_read_unlock_special().

You agree that an interrupt preempting the rcu_read_unlock() is causing
the issues correct? But it is also contained within rcu_read_unlock().
That is, we just don't want interrupts or softirqs from calling the
special function when it preempted rcu_read_unlock().

How about this patch? (again totally untested and not even compiled)

-- Steve

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 7784bd2..0bdf0ea 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -46,6 +46,8 @@
#include <linux/module.h>
#include <linux/hardirq.h>

+DEFINE_PER_CPU(int, in_rcu_read_unlock);
+
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_key;
struct lockdep_map rcu_lock_map =
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 14dc7dd..a4adbb7 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -375,6 +375,8 @@ static void rcu_read_unlock_special(struct task_struct *t)
}
}

+DECLARE_PER_CPU(int, in_rcu_read_unlock);
+
/*
* Tree-preemptible RCU implementation for rcu_read_unlock().
* Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
@@ -386,12 +388,16 @@ void __rcu_read_unlock(void)
{
struct task_struct *t = current;

+ get_cpu_var(in_rcu_read_unlock)++;
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 &&
+ __get_cpu_var(in_rcu_read_unlock) == 1 &&
unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
rcu_read_unlock_special(t);
+ __get_cpu_var(in_rcu_read_unlock)--;
+ put_cpu_var(in_rcu_read_unlock);
#ifdef CONFIG_PROVE_LOCKING
WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
#endif /* #ifdef CONFIG_PROVE_LOCKING */



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