OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Shader Memory Control Functions

Shaders of all types may read and write the contents of textures and buffer objects using image variables. While the order of reads and writes within a single shader invocation is well defined, the relative order of reads and writes to a single shared memory address from multiple separate shader invocations is largely undefined. The order of memory accesses performed by one shader invocation, as observed by other shader invocations, is also largely undefined but can be controlled through memory control functions.

void memoryBarrier();

Controls the ordering of memory transactions issued by a single shader invocation.

void memoryBarrierAtomicCounter();

Controls the ordering of accesses to atomic-counter variables ...