[Ovmsdev] Metrics & Notifications: thread safety

[Michael Balzer]

Jakob,

bool, int and float are 32 bit values, so reads and writes are atomic on ESP32. A read-modify-write cycle would not be atomic, but the metrics API does not
provide that kind of operation.

The effect of SetModified(false) is not to reset the modified bits, it just updates m_defined & m_stale. So this does not cancel out a concurrent
SetModified(true). However, the current implementation does not guarantee NotifyModified() will happen at most once for the same change.

A solution could be to decouple NotifyModified() from SetModified(), possibly moving it to a dedicated task as well. I've been a little bit discomforted since
the beginning with metrics listeners being called within the modifying context. Neither the metric modifier knows about the listener time and stack
requirements, nor does the listener know about the restrictions of the current context. For example, the transmission of the metric change via network can run
within the CAN RX context. Even the creation of a possibly complex notification can be coupled directly to a metrics change.

So maybe decoupling the callback execution is the next step here.

Regards,
Michael


Am 23.01.19 um 16:28 schrieb Jakob Löw:
> Hey,
>
> OvmsMetricBool, -Int and -Float don't seem to use a mutex, but afaik
> there is no garantuee for them to be atomic. A simple fix would be to
> make m_value a std::atomic<float> etc. or use a mutex just as with
> OvmsMetricString.
> Furthermore there might be a small race condition when two tasks modify
> a metric. Assuming the following base situation:
>
> OvmsMetricString dummy("dummy");
> dummy.SetValue("a");
>
> // some other code...
>
> dummy.SetValue("a");
> // now between ovms_metrics.cpp line 793 and 794 the thread is interrupted and this happens:
>
> dummy.SetValue("b");
> // the value is modified, all change listener are called but none resets the modified flag.
>
> // now execution continues with line 794 of dummy.SetValue("a"); from above
> // which essentially does SetModified(false);
> // but in fact the current value is "b" so it was modified.
>
> This also applies to all other metric types, as they all seem to call
> SetModified after changing the value.
> The easy fix would be to put the SetModified into the mutex lock, but
> that would result in a deadlock when one of the change listeners calls
> SetValue.
> I don't see an better way to avoid that race condition though :(
>
> - Jakob
>
> On Tue, 2019-01-22 at 15:49 +0100, Michael Balzer wrote:
>> https://github.com/openvehicles/Open-Vehicle-Monitoring-System-3/comm
>> it/8003fc0da759d5d33682c54cbf38840823477fbc
>>
>> Brain surgery. A second pair of eyes on this would be good.
>>
>> Regards,
>> Michael
>>
>> _______________________________________________
>> OvmsDev mailing list
>> OvmsDev at lists.openvehicles.com
>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev

-- 
Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26

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