static struct hlist_head *pid_hash;
hlist_head is a kernel standard data element used to create doubly linked hash lists (Appendix C describes the structure of such lists and introduces several auxiliary functions for processing them).
pid_hash is used as an array of hlist_heads. The number of elements is determined by the RAM configuration of the machine and lies between 24 = 16 and 212 = 4,096. pidhash_init computes the apt size and allocates the required storage.
Suppose that a new instance of struct pid has been allocated and set up for a given ID type type. It is attached to a task structure as follows:
Continue reading here: Kernelpidc
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