Re: INFO: possible circular locking dependency detected

[Steven Rostedt <rostedt@goodmis.org>]

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 */