Re: [PATCH] Remove never-migrates assumption from rcu_process_callbacks()

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> Hello again!
> 
> This patch fixes a potentially invalid access to a per-CPU variable in 
> rcu_process_callbacks().  This per-CPU access needs to be done in such 
> a way as to guarantee that the code using it cannot move to some other 
> CPU before all uses of the value accessed have completed.  Even though 
> this code is currently only invoked from softirq context, which 
> currrently cannot migrate to some other CPU, life would be better if 
> this code did not silently make such an assumption.

i've got the patch below queued up already - same thing, right?

	Ingo

----------->
Subject: rcupreempt: fix hibernate/resume in presence of PREEMPT_RCU and hotplug
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date: Wed, 27 Feb 2008 16:21:10 -0800

This fixes a oops encountered when doing hibernate/resume in presence
of PREEMPT_RCU.  The problem was that the code failed to disable preemption
when accessing a per-CPU variable.  This is OK when called from code
that already has preemption disabled, but such is not the case from
the suspend/resume code path.

Reported-by: Dave Young <hidave.darkstar@gmail.com>
Tested-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/rcupreempt.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux/kernel/rcupreempt.c
===================================================================
--- linux.orig/kernel/rcupreempt.c
+++ linux/kernel/rcupreempt.c
@@ -918,8 +918,9 @@ void rcu_offline_cpu(int cpu)
 	 * fix.
 	 */
 
+	local_irq_save(flags);
 	rdp = RCU_DATA_ME();
-	spin_lock_irqsave(&rdp->lock, flags);
+	spin_lock(&rdp->lock);
 	*rdp->nexttail = list;
 	if (list)
 		rdp->nexttail = tail;