Locking while executing bottom-halves and process context code

Hello, While reading a section in Linux Kernel Development, I came across the following: "If process context code and a bottom half share data, you need to disable bottom-half processing and obtain a lock before accessing the data." Why is this the case? Can one not disable/lock the process context code instead of the bottom-half and access data? Similarly, for the statement, "If interrupt context code and a bottom half share data, you need to disable interrupts and obtain a lock before accessing the data." Any help in clarifying this would be much appreciated. Thanks, Pratyush