In operating systems, write barrier is a mechanism for enforcing a particular ordering in a sequence of writes to a storage system in a computer system. For example, a write barrier in a file system is a mechanism (program logic) that ensures that in-memory file system state is written out to persistent storage in the correct order.
In Garbage collection
A write barrier in a garbage collector is a fragment of code emitted by the compiler immediately before every store operation to ensure that (e.g.) generational invariants are maintained. A write barrier in a memory system, also known as a memory barrier, is a hardware-specific compiler intrinsic that ensures that all preceding memory operations "happen before" all subsequent ones.
References
Write barrier Wikipedia(Text) CC BY-SA