if i play Baron in a controlled battlefield and my legend is riven The Exile. Did Baron creat the Pit AND enters ready?

Yes — you get both. Baron's "add the Baron Pit if it's not there already, if you do I enter there" and The Exile's "units you play to battlefields you control enter ready" are both replacement effects that apply to the same enter-the-board event, so as the controller of Baron you choose the order they apply (CR 372, CR 369.3).

Order them so the ready one resolves first and then the Pit relocation: the relocation only rewrites where he enters, it doesn't change the state he enters in. Riven's effect keys off where he was played to (a battlefield you control), which is true, so he still enters ready even though he ends up in the Pit.

Note the Pit arrives uncontrolled, so it becomes Contested and you'll go through a Non-Combat Showdown, then Conquer the Pit after it ends (assuming you haven't already scored it that turn). The one way to miss it: playing him somewhere other than a battlefield you control, e.g. to your Base — then The Exile's condition isn't met and he enters exhausted.