The following warning from the reIndex command documentation is not accurate.

This command obtains a write lock on the affected database and will block other operations until it has completed.

The warning is only true sometimes. The reIndex command will hold a lock and block all other database operations if it is rebuilding _id index. As for all other indexes, the options that were used initially to create the index would be used to rebuild the index. For example, if ensureIndex background option were set to true, then the same option would be true for reIndex, which does not block other database activities. Conversely, if the index were build in the foreground, then reIndex would be performed in the foreground.