Re: [PATCH] eliminte NMI entry/ exit code

[George Anzinger <george@mvista.com>]

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/