RC phase shift oscillator : how to give positive feedback?
Your premise is incorrect: "a oscillator requires a positive feedback".

There are two requirements for an amplifier circuit to oscillate:


The overall ("loop") feedback must be 0 degrees ( or a multiple thereof, e.g. 360, 720 degrees).
There must be enough overall gain to start &amp; sustain oscillation. (This is sometimes states as "The loop gain must be greater than 1.")


The prior commentor suggested: "the signal will have been inverted once more by the RCs". Not true. RC networks cannot "invert" a signal. They can only delay or phase shift it. For practical purposes phase shifting a signal by 180 degrees is nearly equivalent to "inverting" the signal because true inversion ( as thru an inverting amplifier) also results in a virtual phase shift of 180 degrees - but only for symetrical waveforms like sine waves, triangle wave and square waves, but not for complex waves like musical notes and voice signals. Be mindful of this distinction when analyzing oscillators and amplifiers.

One RC network can phase shift a sine wave by 0 to 90 degrees. In the act of phase shiting the signal it also reduces the amplitude of the signal. Since 90 degrees phase shift is at the outer limits of an RC's phase shifting ability, we can't use just two RCs. So, we stack three RC networks in series and allow each to contribute 60 degrees of phase shift, for a total phase shift of 180 degrees ( but at only one specific frequency!). These three series-connected RC networks will also attentuate the signal signifcantly at the specific frequency. That attentuation has to be made up in the gain of the amplifier in order to meet the oscillation requirement #2 of having a loop gain greater than 1. 

It's also possible to make oscillators using 4,5,6 or more series connected RC's. But there's usually no good reason to take this design approach.

The inversion of the amplifier provides the other 180 degrees of "phase shift". So we have a total loop feedback of 360 degrees or 0 degrees, depending on how you look at it. Either case will meet oscillation requirement #1.

By the way, these RC oscialltors are not very good at making pure sinusoidal outputs. To get the most-sinusoidal-like output you must keep the gain of the amplifier itself low enough so it just exceeds that necessary to maintain oscillation (i.e. 1.000) . Make it too high and the output will be a clipped sine wave, often approaching a square wave in appearance. Experiment with the gain to get both a near sinusoidal output and ensure the oscillator will start reliably. The lower the amplifier's gain, the slower and less reliably the oscillator will properly start up. Power the oscillator on and off a number of times to ensure it will reliably start every time power is applied. It's also a good idea to observe the envelope of the oscillation on an oscilloscope as you apply power to the circuit. You should see a nice exponential increase of the envelope starting immediately at power application, with no drop-outs or squiggles.