When the Linux kernel removes an entry from a linked list using list_del(), it doesn't set its next and prev pointers to NULL. On typical 64-bit systems, it sets them to 0xdead000000000100 and 0xdead000000000122 (LIST_POISON1 and LIST_POISON2). these deliberately invalid addresses help catch code that accidentally accesses a removed list entry. Instead of silently following a stale pointer, the kernel can fault at a recognizable poison address. It's called pointer poisoning.