linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Tue, 2 Dec 2014 12:56:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Dâniel Fraga <fragabr@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Tue, Dec 02, 2014 at 05:24:39PM -0200, Dâniel Fraga wrote:
> On Tue, 2 Dec 2014 11:11:43 -0800
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > OK.  I need to know exactly what version of the Linux kernel you are
> > using.  3.18-rc7?  (I am not too worried about exactly which version
> > you are using as long as I know which version it is.)
> 
> 	Ok, I stopped bisecting and went back to 3.17.0 stock kernel.
> I'm testing with 3.17.0 kernel because this one is the first to show
> problems. If you want me to go to 3.18-rc7, just ask I can checkout
> through git.
> 
> 	Ps: my signature will reflect the kernel I'm using now ;)

And I left out a step.  Let's make sure that my preempt_disabled() hack
to CONFIG_TREE_PREEMPT_RCU=y has the same effect as the Kconfig hack
that allowed CONFIG_PREEMPT=y and CONFIG_TREE_PREEMPT_RCU=n.  Could you
please try out the following patch configured with CONFIG_PREEMPT=y
and CONFIG_TREE_PREEMPT_RCU=y?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index e0d31a345ee6..fff605a9e87f 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -71,7 +71,11 @@ module_param(rcu_expedited, int, 0);
  */
 void __rcu_read_lock(void)
 {
-	current->rcu_read_lock_nesting++;
+	struct task_struct *t = current;
+
+	if (!t->rcu_read_lock_nesting)
+		preempt_disable();
+	t->rcu_read_lock_nesting++;
 	barrier();  /* critical section after entry code. */
 }
 EXPORT_SYMBOL_GPL(__rcu_read_lock);
@@ -92,6 +96,7 @@ void __rcu_read_unlock(void)
 	} else {
 		barrier();  /* critical section before exit code. */
 		t->rcu_read_lock_nesting = INT_MIN;
+		preempt_enable();
 		barrier();  /* assign before ->rcu_read_unlock_special load */
 		if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special.s)))
 			rcu_read_unlock_special(t);

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