[dbcfd]

The only time I could see that not being threadsafe is with something like a double on a system without native support, such that bits could change during operation. Even then I think the information gets pulled out.

On most systems, the value of calc will still be a race condition dependent on what thread gets there first, even if you lock around m1 or make it atomic. Calc will pull the value from m1 into a register, perform the operation, then return the newly calculated value. Another thread changing m1 will not matter.