Answer 7—Read-modify-write operations must be protected by additional access controls only when more than one process can write to the variable in question. If only one process writes to a particular variable, no additional controls are required.

In the case of the simple queue mentioned previously, only the sending process writes to the data array and to the head index. Only the receiving process writes to the tail index. No additional access controls are needed on either index.

On the other hand, if there were multiple sending processes, then the head index would need a mutex. And if there were multiple receiving processes, the tail index would need one.

Contributor: David Tweed