Please look at memory consistency section in OpenCL Spec
Let me paste it from OpenCL 1.2 spec for your reference:
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Within a work-item memory has load / store consistency. Local memory is consistent across
work-items in a single work-group at a work-group barrier. Global memory is consistent across
work-items in a single work-group at a work-group barrier, but there are no guarantees of
memory consistency between different work-groups executing a kernel.
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I hope this answers your question