Re: [Xenomai-help] Event flag anomaly

[Philippe Gerum <rpm@xenomai.org>]

ROSSIER Daniel wrote:
> 
> Hi,
> 
> I've some problems to understand how the event flag semantic is defined.
> 
> In the example below, I just defined a RT_EVENT task which waits on event mask 1.
> (Simply bit 1 has to be set). 
> When I do a rt_event_signal(&meteoEvents, 1) from another task, the blocking task
> is logically waked up. Fine. On the next iteration, the event bit set is remaining
> unchanged and the rt_event_wait() doesn't wait anymore since the event is activated.
> (please consider the rt_event_clear as commented lines as it is). Well, it seems
> OK. BUT, on the third iteration, the rt_event_wait() is blocking! Actually, the bit
> set has been cleared in the meanwhile.
> 
> Looking at the rt_event_wait() code in native/event.c, it effectively turns out that
> the bit set is cleared according to the mask, if and only if the task doesn't need to wait.
> 
> Please could you explain me what's the rationale behind this strange behaviour? I would propose that the bits
> remain unchanged until a clear().  (I probably missed something in my reasoning).

You are right. Auto-clearing the satisfied event bits is spurious here. Fixed, thanks.

> 
> Here is the example:
> ---------------------------
> 
> void meteoDataAcquisition(void *cookie) {
> 
> unsigned long receivedMask;
> 
>   while(1) {
> 
>     rt_event_wait(&meteoEvents, 1, &receivedMask, EV_ANY, TM_INFINITE);
>     //rt_event_clear(&meteoEvents, 1, &receivedMask);
>     
>     
>     printk("#Task activated\n");
>    
>   }
>   
> }
> -------------------------------
> 
> And a piece of code from event.c
> 
> ....
>  if (((mode & EV_ANY) && (mask & event->value) != 0) ||
>         (!(mode & EV_ANY) && ((mask & event->value) == mask)))
>         {
>         *mask_r = (event->value & mask);
>         event->value &= ~mask;        --> this is not done after the thread has been waked up.
>         goto unlock_and_exit;
>         }
> .....
> 
> 
> 
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-- 

Philippe.