If i remember correctly Xperia M can only be partially rooted because of locked boot loader. You can neither install any recovery or custom rom on the device due to locked boot loader. Now, i don’t have Xperia M anymore but if you have it and want to unlock its full potential, you can try to do so. Just don’t blame me if anything happens to it though, as usual. Safety first.
And by the way, it will void your warranty. It is clearly stated on the official page. If you have Xperia M or any other Sony device that you’d like to unlock the boot loader of, just go this official Sony’s developer page and select your device in that drop down list. Before doing anything just enter this code *#*#7378423#*#*(same for all devices) in your phone’s dialler to check if its boot loader can be unlocked or not. You can see more about that in the image below. 
Click continue, it will ask you to enter your e-mail address for verification. To continue further you’d have to log-in to your e-mail account and click on the link that they’ve sent you. It will take you to a page which will ask for your the IMEI number of your device. Keep in mind that the IMEI number you’ve entered will be saved in-to Sony’s database. So, if anything happens to your device by unlocking the boot loader or during the process. They will know that you’ve been messing around the device and might charge you for the in house repair/service.
After you enter a correct IMEI number of your device it will assign you a unlock code. It will look something like in the image below. That is my unlock code it is related/connected to the IMEI number of my device. It’s for reference only, it won’t work with any other device. 
This is a 4 step process and this is the fourth and last step and it is also the most technical one. I would suggest you to restrain from continuing at this point if you don’t know what you are doing. Because if you try anything funny during this step it might really mess up your device. It’s better if you read about it in detail, just check out both the images below. Read carefully.
Neither me nor Sony would be responsible for the following:
- If you end up with a bricked device.
- If something unimaginable happens to your device.
- If your device starts acting weird.
- Any case of act of God or Satan.
- If Nokia 3310 decides to sit on it.
You can ask me for friendly advice or suggestion though, if you are confused about something or stuck at somewhere. Shoot your questions in comments section below. Just don’t blame me if you end up with a dead device! Like friends usually do! Sorry, can’t help it. Have to be careful all the time you know.