[Dune] [#737] Wrong results in concurrent access to reference elements

[Dune]

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User who did this - Martin Weiser (weiser) 

Attached to Project - Dune
Summary - Wrong results in concurrent access to reference elements
Task Type - Bug Report
Category - Grid
Status - New
Assigned To - 
Operating System - Unspecified / All
Severity - Medium
Priority - Normal
Reported Version - SVN (pre2.0)
Due in Version - Undecided
Due Date - Undecided
Details - The mapping of subentity-coordinates to reference-coordinates in ReferenceElement<ctype,dim>::global() uses a static FieldMatrix (in mapGlobal(), referenceelements.hh:34).

In multithreaded applications, this leads to wrong results due to concurrent access to the same matrix,
which is shared by all threads due to being static.

I propose to remove the static qualifier. 

The impact on performance should be negligible because
the construction of a FieldMatrix on the stack is fast. The only issue might be that the constructor initializes the matrix elements to zero. Using a static matrix saves this initialization. However, a good optimizer should detect the double write and remove the initialization.

More information can be found at the following URL:
http://www.dune-project.org/flyspray/index.php?do=details&task_id=737

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