[PATCH] eliminte NMI entry/ exit code

[PATCH] eliminte NMI entry/ exit code

[George Anzinger]

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The NMI entry and exit code fiddles with bits in the preempt count.  If 
an NMI happens while some other code is doing the same, bits will be 
lost.  This patch removes this modify code from the NMI path till we can 
come up with something better.
-- 
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)

Re: [PATCH] eliminte NMI entry/ exit code

[Nick Piggin]

George Anzinger wrote:
> The NMI entry and exit code fiddles with bits in the preempt count.  If 
> an NMI happens while some other code is doing the same, bits will be 
> lost.  This patch removes this modify code from the NMI path till we can 
> come up with something better.
> 

Humour me for a minute here...
NMI restores preempt_count back to its old value upon exit, right?
So what does a race case look like?

Nick

-- 
SUSE Labs, Novell Inc.

Send instant messages to your online friends http://au.messenger.yahoo.com 

Re: [PATCH] eliminte NMI entry/ exit code

[George Anzinger]

Nick Piggin wrote:
> George Anzinger wrote:
> 
>> The NMI entry and exit code fiddles with bits in the preempt count.  
>> If an NMI happens while some other code is doing the same, bits will 
>> be lost.  This patch removes this modify code from the NMI path till 
>> we can come up with something better.
>>
> 
> Humour me for a minute here...
> NMI restores preempt_count back to its old value upon exit, right?
> So what does a race case look like?

Normal code                   NMI
fetch preempt_count
add                   <-----  interrupt here add and store then subtract 
and store, darn!
store preempt_count

Ok, no problem.

The problem is in the RT code when PREEMPT_DEBUG is on.  The tests for 
reasonable counts fail because of the rather undefined state when NMI 
picks up the word.  The failure is on the NMI side...
> 

-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/

Re: [PATCH] eliminte NMI entry/ exit code

[Zachary Amsden]

George Anzinger wrote:

> Nick Piggin wrote:
>
>> George Anzinger wrote:
>>
>>> The NMI entry and exit code fiddles with bits in the preempt count.  
>>> If an NMI happens while some other code is doing the same, bits will 
>>> be lost.  This patch removes this modify code from the NMI path till 
>>> we can come up with something better.
>>>
>>
>> Humour me for a minute here...
>> NMI restores preempt_count back to its old value upon exit, right?
>> So what does a race case look like?
>
>
> Normal code                   NMI
> fetch preempt_count
> add                   <-----  interrupt here add and store then 
> subtract and store, darn!
> store preempt_count
>
> Ok, no problem.
>
> The problem is in the RT code when PREEMPT_DEBUG is on.  The tests for 
> reasonable counts fail because of the rather undefined state when NMI 
> picks up the word.  The failure is on the NMI side... 


So NMI changing the preempt count and restoring in the middle of a RWM 
is not the problem.  Thus I don't understand what the issue is.  NMI 
must undo all side effects.  Does the PREEMPT_DEBUG code check the count 
somewhere within the NMI handler?  If so, shouldn't the proper fix be to 
make that code aware that it could be running inside of an NMI and/or 
ensure that code is not called from within the NMI handler?

Zach

Re: [PATCH] eliminte NMI entry/ exit code

[George Anzinger]

Zachary Amsden wrote:
> George Anzinger wrote:
> 
>> Nick Piggin wrote:
>>
>>> George Anzinger wrote:
>>>
>>>> The NMI entry and exit code fiddles with bits in the preempt count.  
>>>> If an NMI happens while some other code is doing the same, bits will 
>>>> be lost.  This patch removes this modify code from the NMI path till 
>>>> we can come up with something better.
>>>>
>>>
>>> Humour me for a minute here...
>>> NMI restores preempt_count back to its old value upon exit, right?
>>> So what does a race case look like?
>>
>>
>>
>> Normal code                   NMI
>> fetch preempt_count
>> add                   <-----  interrupt here add and store then 
>> subtract and store, darn!
>> store preempt_count
>>
>> Ok, no problem.
>>
>> The problem is in the RT code when PREEMPT_DEBUG is on.  The tests for 
>> reasonable counts fail because of the rather undefined state when NMI 
>> picks up the word.  The failure is on the NMI side... 
> 
> 
> 
> So NMI changing the preempt count and restoring in the middle of a RWM 
> is not the problem.  Thus I don't understand what the issue is.  NMI 
> must undo all side effects.  Does the PREEMPT_DEBUG code check the count 
> somewhere within the NMI handler?  If so, shouldn't the proper fix be to 
> make that code aware that it could be running inside of an NMI and/or 
> ensure that code is not called from within the NMI handler?

Yes that is the problem.  The sanity check in PREEMPT_DEBUG fails when 
called from the NMI handler.
> 


-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/

Re: [PATCH] eliminte NMI entry/ exit code

[Linus Torvalds]

On Fri, 12 Aug 2005, George Anzinger wrote:
>
> The NMI entry and exit code fiddles with bits in the preempt count.  If 
> an NMI happens while some other code is doing the same, bits will be 
> lost.

Why?

Even if an NMI happens in the middle of a read-modify-write sequence that 
is critical, the NMI exit is supposed to undo whatever it was that the NMI 
entry did, so the preempt counters are "safe" wrt NMI: they may change, 
but they always change back by the time anybody cares.

This, btw, is something we depend on wrt _normal_ interrupts too. It's why 
people can read/modify/write preempt count without having to disable 
interrupts.

			Linus