Re: [PATCH 1/3] x86: only call smp_processor_id in non-preempt cases

[Don Zickus]

On Thu, Nov 18, 2010 at 03:49:21PM +0100, Ingo Molnar wrote:
> 
> * Don Zickus <dzickus@xxxxxxxxxx> wrote:
> 
> > On Thu, Nov 18, 2010 at 09:14:07AM +0100, Ingo Molnar wrote:
> > > 
> > > * Don Zickus <dzickus@xxxxxxxxxx> wrote:
> > > 
> > > > There are some paths that walk the die_chain with preemption on.
> > > 
> > > What are those codepaths? At minimum it's worth documenting them.
> > 
> > Well the one that caused the bug was do_general_protection which walks the
> > die_chain with DIE_GPF.
> > 
> > I can document them, though it might be time consuming to audit them and hope they 
> > don't change.
> 
> Listing one example is enough.
> 
> > [...]  I guess my bigger question is, is it expected that anyone who calls the 
> > die_chain to have preemption disabled?  If not, then does it matter if we document 
> > it?
> 
> Yes, it might be a bug to call those handlers with preemption on (or even with irqs 
> on). But if the code is fine as-is then documenting a single example would be nice.
> 

Is this better?

Cheers, 
Don

------------------------------------->
From: Don Zickus <dzickus@xxxxxxxxxx>
Date: Mon, 1 Nov 2010 13:34:33 -0400
Subject: [PATCH 1/6] x86: only call smp_processor_id in non-preempt cases

There are some paths that walk the die_chain with preemption on.
Make sure we are in an NMI call before we start doing anything.

This was triggered by do_general_protection calling notify_die with
DIE_GPF.

Reported-by: Jan Kiszka <jan.kiszka@xxxxxx>
Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx>
---
 arch/x86/kernel/apic/hw_nmi.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 5c4f952..ef4755d 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -49,7 +49,7 @@ arch_trigger_all_cpu_backtrace_handler(struct notifier_block *self,
 {
 	struct die_args *args = __args;
 	struct pt_regs *regs;
-	int cpu = smp_processor_id();
+	int cpu;
 
 	switch (cmd) {
 	case DIE_NMI:
@@ -60,6 +60,7 @@ arch_trigger_all_cpu_backtrace_handler(struct notifier_block *self,
 	}
 
 	regs = args->regs;
+	cpu = smp_processor_id();
 
 	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
 		static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
-- 
1.7.3.2

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