Most parallel algorithms provided by Intel® Threading Building Blocks (Intel® TBB) are generic. They operate on all types that model the necessary concepts. Parallel algorithms may be nested. For example, the body of a parallel_for can invoke another parallel_for.
When the body of an outer parallel algorithm invokes another parallel algorithm, it may cause the outer body to be re-entered for a different iteration of the outer algorithm.
For example, if the outer body holds a global lock while calling an inner parallel algorithm, the body will deadlock if the re-entrant invocation attempts to acquire the same global lock. This ill-formed example is a special case of a general rule that code should not hold a lock while calling code written by another author.