Comment by pjc50

This is one of those "everyone doing this kind of work knows" that's rather hard to source, but: this is basically the point of volatile. Especially for reads rather than writes, where you may want to read some location that is being written into by a different piece of hardware.

People used to use it for thread synchronization before proper memory barrier primitives (see https://mariadb.org/wp-content/uploads/2017/11/2017-11-Memor... ) were available. It was not entirely reliable for this purpose.