Enable viewThreadSafe on dune-grid

A quick debug on a 2D uggrid with only triangles leads me to a data race on writes to the usage statistics ce_usage[MAX_CONTROL_ENTRIES]. After commenting writes to these statistics, I was able to perform parallel loops without data-races.

It would be very nice if these statistics collection were optional in order to enable viewThreadSafe at least on simplices for 2D. If this sounds interesting to anybody, I could also check the other cases.

It would be very nice if these statistics collection were optional in order to enable viewThreadSafe at least on simplices for 2D. If this sounds interesting to anybody, I could also check the other cases.

why so you suggest making them optional? One could simply make them protect them via lock. Could you point to the critical code section?

What kind of lock? A mutex sounds like an overkill. If someone needs that one could make it atomic, it’s just a counter, but I would avoid the hassle.

We are talking about writes to the ce_used in the methods ReadCW and WriteCW.

In didn't even know this statistics collection existed. Is it of interest to anyone? Otherwise we may as well rip it out.

It looks like an internal variable since no extern keyword is used, however, is not used anywhere in the library. Seems safe to remove. I will make a MR for that.

changed the description

created merge request !198 (merged) to address this issue

mentioned in merge request !198 (merged)

mentioned in commit 71a2151b

closed with merge request !198 (merged)

mentioned in issue core/dune-grid#138 (closed)