Re: [riot-devel] xtimer race condition

[Joakim Nohlgård]

Hi Matthew,
My guess is that you are running that function with interrupts
disabled, which blocks the xtimer timeouts. The reason that short
sleeps work in xtimer is that internally that is handled as a spin,
while longer timeouts are handled by setting a hardware timer target.
You can try using xtimer_spin instead of usleep, but chances are your
PR will not be merged if you are using xtimer_spin with long timeouts.
xtimer_spin is a busy wait which should be used only for very short
periods, since it will be obstructing other parts of the system from
running while you are spinning.

Best regards,
Joakim

On Sat, Mar 31, 2018 at 8:36 AM, Matthew Blue
 wrote:
> Hello all,
>
> I have had some odd problems with a PR I am attempting to finalize:
>
> cpu/atmega_common: RTT and RTC support
> https://github.com/RIOT-OS/RIOT/pull/8842
>
> When working with the real-time timers, there are a number of times that
> you have to wait a few of the slow clock cycles to go by. I am using
> xtimer_usleep to sleep the thread temporarily. But! xtimer causes the
> code I am using to hang. Debugging it is difficult, because anything
> sending debug messages over UART causes the problems to disappear!
>
> I have been editing and running various numbers:
> xtimer_usleep(8); 8 and bigger fail
> xtimer_usleep(7); 7 and smaller work
>
> Bigger numbers make them fail!
>
> Here is a snippet of the offending code:
>
>> void __asynch_wait(uint8_t num_cycles) {
>> /* Wait until all busy flags clear */
>> while( ASSR & ((1 << TCN2UB) | (1 << OCR2AUB) | (1 << OCR2BUB)
>>  | (1 << TCR2AUB) | (1 << TCR2BUB)) ) {
>> #ifdef MODULE_XTIMER
>> /* Sleep for num_cycles RTC cycles */
>> xtimer_usleep(num_cycles * 35 * US_PER_MS);
>> #else
>> /* Suppress unused parameter warning */
>> (void)num_cycles;
>>
>> thread_yield();
>> #endif
>> }
>> }
>
> Anyone have any ideas on what is going on?
>
> Sincerely,
> Matthew
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[riot-devel] xtimer race condition

[Matthew Blue]

Hello all,

I have had some odd problems with a PR I am attempting to finalize:

cpu/atmega_common: RTT and RTC support
https://github.com/RIOT-OS/RIOT/pull/8842

When working with the real-time timers, there are a number of times that
you have to wait a few of the slow clock cycles to go by. I am using
xtimer_usleep to sleep the thread temporarily. But! xtimer causes the
code I am using to hang. Debugging it is difficult, because anything
sending debug messages over UART causes the problems to disappear!

I have been editing and running various numbers:
xtimer_usleep(8); 8 and bigger fail
xtimer_usleep(7); 7 and smaller work

Bigger numbers make them fail!

Here is a snippet of the offending code:

> void __asynch_wait(uint8_t num_cycles) {
> /* Wait until all busy flags clear */
> while( ASSR & ((1 << TCN2UB) | (1 << OCR2AUB) | (1 << OCR2BUB)
>  | (1 << TCR2AUB) | (1 << TCR2BUB)) ) {
> #ifdef MODULE_XTIMER
> /* Sleep for num_cycles RTC cycles */
> xtimer_usleep(num_cycles * 35 * US_PER_MS);
> #else
> /* Suppress unused parameter warning */
> (void)num_cycles;
> 
> thread_yield();
> #endif
> }
> }

Anyone have any ideas on what is going on?

Sincerely,
Matthew
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