Re: [PATCH] eliminte NMI entry/ exit code

[George Anzinger <george@mvista.com>]

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/