在32位的機器上對long型變量進行加減操作存在並發隱患的說法是正確的。
原因就是：線程切換帶來的原子性問題。
非volatile類型的long和double型變量是8字節64位的，32位機器讀或寫這個變量時得把人家咔嚓分成兩個32位操作，可能一個線程讀了某個值的高32位，低32位已經被另一個線程改了。所以官方推薦最好把long\double 變量聲明為volatile或是同步加鎖synchronize以避免並發問題。
貼一段java文檔說明：
https://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html#jls-17.7
17.7. Non-Atomic Treatment of double and long
For the purposes of the Java programming language memory model, a single write to a non-volatile long or double value is treated as two separate writes: one to each 32-bit half. This can result in a situation where a thread sees the first 32 bits of a 64-bit value from one write, and the second 32 bits from another write.
Writes and reads of volatile long and double values are always atomic.
Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values.
Some implementations may find it convenient to divide a single write action on a 64-bit long or double value into two write actions on adjacent 32-bit values. For efficiency's sake, this behavior is implementation-specific; an implementation of the Java Virtual Machine is free to perform writes to long and double values atomically or in two parts.
Implementations of the Java Virtual Machine are encouraged to avoid splitting 64-bit values where possible. Programmers are encouraged to declare shared 64-bit values as volatile or synchronize their programs correctly to avoid possible complications.