Fix a rather rare race condition in MT mode: we checked that we did not yet visit a face by looking at a value that is written when visiting this face. We double checked later when writing into it. In some rare conditions, it could happen that another process was working on it, but only wrote between the two checks, making the latter fail.
Fix a rather rare race condition in MT mode: we checked that we did
not yet visit a face by looking at a value that is written when
visiting this face. We double checked later when writing into it. In
some rare conditions, it could happen that another process was working
on it, but only wrote between the two checks, making the latter fail.


git-svn-id: 31d9d2f6432a47c86a3640814024c107794ea77c@4910 0785d39b-7218-0410-832d-ea1e28bc413d

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