[Libmesh-users] Parallel::/Threads:: interaction

[Roy S. <roy...@ic...>]

Thoughts?  Right now we only call Parallel:: functions from thread 0,
so we should be safe even on non-thread-safe MPI stacks.

But should that change?  And what should users be allowed to do?

Most of the Parallel:: functions would break right now if called from
more than one thread at once.  It would be possible in theory to
change that - e.g. Parallel::max() would first take the max over all
threads, then take the max over all ranks, then distribute the max
back to all threads - but it would be pretty difficult in practice,
particularly since it's somewhat contrary to the task-based paradigm
in TBB.

I'd like to instead just "enforce" the current restrictions - set a
Threads::_now_multithreaded bool to true at the start of and false at
the end of any Threads::parallel_* call, and then do a
assert(!Threads::now_multithreaded()) in every Parallel:: call.

Thoughts?
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Roy