[Tinyos-host-mote-wg] Re: [Tinyos-2.0wg] Timer.fired

[Philip Levis]

On May 19, 2005, at 8:14 PM, Cory Sharp wrote:

> On 5/19/05, Joe Polastre <joe.polastre at gmail.com> wrote:
>
>> Except if an underlying interrupt occurred from an Alarm while I was
>> asking (in task context) how many timers I missed.
>>
>
> I don't understand.  What are you and Phil thinking is going to happen
> in this case that's bad?  An interrupt occurs and another task is
> posted...
>

numMissed and when are stored in some internal state variables, then  
passed in the firing task. Here's the situation I see:

interrupt fires, variables set, task posted
task starts running, fired event calls howMany() command
interrupt fires, variables changed, task posted (but fails, already  
posted)
task continues running, calls when() command

The results of when() and howMany() could be inconsistent.

It may be that this is not an issue in your implementation, but it  
could be an issue in another implementation; interface itself has the  
possibility of race conditions, because it's something that sits on  
top of an interrupt source.

Phil



-------

"We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time."

- T. S. Eliot,  'Little Gidding'

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