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* GLSL : common mistakes#Sampling and Rendering to the Same Texture
NV_texture_barrier might be useful to you on NVidia specifically, but I don't know of a cross-vendor way to support this.
OpenCL IMO is a non-starter except for limited use cases, as IIRC flipping back and forth requires a full pipeline flush/sync (that is, in the absense of cl_khr_gl_event / ARB_cl_event). An OpenGL Compute Shader is much more interesting in terms of avoiding that overhead, but I'm not an expert on those yet.