Re: [patch] Real-Time Preemption, -RT-2.6.13-rc4-V0.7.53-01, High Resolution Timers & RCU-tasklist features

[George Anzinger <george@mvista.com>]

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Ingo, all

I, silly person that I am, configured an RT, SMP, PREEMPT_DEBUG system. 
  Someone put code in the NMI path to modify the preempt count which, 
often as not will generate a PREEMPT_DEBUG message as there is no tell 
what state the preempt count is in on an NMI interrupt.  I have sent the 
attached patch to Andrew on this, but meanwhile, if you want RT, SMP, 
PREEMPT_DEBUG you will be much better off with this.
-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/

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Source: MontaVista Software, Inc. George Anzinger <george@mvista.com>
Type: Defect Fix 

Description:

    Modifying a word from NMI code runs the very real risk of loosing
    either then new or the old bits.  Remember, we can not prevent an
    NMI interrupt from ANYWHERE, inparticular between the read and the
    write of a read modify write sequence.

    This patch removes the update of the preempt count from the NMI
    path.

Signed-off-by: George Anzinger<george@mvista.com>

 hardirq.h |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

Index: linux-2.6.13-rc/include/linux/hardirq.h
===================================================================
--- linux-2.6.13-rc.orig/include/linux/hardirq.h
+++ linux-2.6.13-rc/include/linux/hardirq.h
@@ -98,9 +98,12 @@ extern void synchronize_irq(unsigned int
 #else
 # define synchronize_irq(irq)	barrier()
 #endif
-
-#define nmi_enter()		irq_enter()
-#define nmi_exit()		sub_preempt_count(HARDIRQ_OFFSET)
+/*
+ * Re think these.  NMI _must_not_ share data words with non-nmi code
+ * Meanwhile, just do a no-op.
+ */
+#define nmi_enter()	/*	irq_enter()  */
+#define nmi_exit()	/*	sub_preempt_count(HARDIRQ_OFFSET) */
 
 #ifndef CONFIG_VIRT_CPU_ACCOUNTING
 static inline void account_user_vtime(struct task_struct *tsk)