The volatile modifier ensures that updates to a variable are propagated predictably to other threads. A read of a volatile variable always returns the most recent write by any thread.
However, this does not mean that all updates to a volatile variable are atomic. For example, if you increment or decrement a volatile variable, you are actually doing (1) a read of the variable, (2) an increment or decrement of a local copy, and (3) a write back to the variable. Each step is atomic individually, but the whole sequence is not, and it will cause a race condition if two threads try to increment or decrement a volatile variable at the same time. The same is true for compound assignment, e.g. foo += bar.
If you intended for this update to be atomic, you should wrap all update operations on this variable in a synchronized block. If the variable is an integer, you could use an AtomicInteger instead of a volatile int.
Suppress false positives by adding the suppression annotation @SuppressWarnings("NonAtomicVolatileUpdate") to the enclosing element.