Why cant a 64-bit value be accessed atomically on a 32-bit system?

Question

Reading upon Linux Scheduler from "https://www.cs.columbia.edu/~smb/classes/s06-4118/l13.pdf" I came across a statement :

"A 64-bit value cannot be accessed atomically on a 32-bit machine"

My question is , why is it so ?

we can use locks to make it atomic, as described in the next bullet on the same article. If thats true, then what is meant by the quoted line.

Answer 1

As per my comment, they probably mean that the load from memory itself isn't atomic.

A 64bit word on a 32bit machine might take two memory accesses to get the full word. During the two reads, a context switch could occur or may be overwritten by a DMA device. Therefore, you get a half correct value.