Overhead shoulder mobility is something every crossfitter should achieve before attempting exercises like overhead squats, hand stand push ups, and even toes to bar. If you have issues with raising your arm over your head to a full 180 degrees, chances are your body is compensating when you do these exercises, putting extra strain on the shoulders, spine and the corresponding muscles (think low back pain, shoulder impingement and rotator-cuff tears).
Ideally, in full shoulder flexion we should be able to see the side of your face from your ear to nose – without having a forward head posture. Here are some common faults with overhead mobility:

Poor Lumbo-pelvic control
As you raise your arms, excessive lumbar extension (low back arch) will cause rib-flare and give a false sense of the overhead position. You’ll want to work on activating your core in the overhead position – grab a PVC pipe and raise it overhead – take a deep breath and contract your abs. This will promote a posterior tilt in your pelvis and help keep your ribcage down (you might notice your arms are lower now). Breathe out slowly and try to push your arms further back.

Tight Muscles
Shortened lats are a big contributor to reduced overhead flexion and can also result inthe hyper-extension/rib flare posture. Tight lats also put your shoulder into an internally rotated position which is a 1 -way ticket to impingement, so you’ll want to stretch them regularly: Grab a band/pole in EXTERNAL ROTATION (thumb up), and sink into the stretch, twisting your pelvis AWAY from the side you’re stretching.
Another exercise you might find useful to fix both of the above: The Kettlebell Pullover.
A couple things to note: We’re focusing on the lowering of the weight with control. Notice the lack of ribcage flaring as the weight goes further back – his core is actively promoting a neutral spine and the movement is solely coming from his shoulders. You can do this on a bench as you get more mobility, the goal is to get your arms up to your ears.

If you’re struggling with achieving a true overhead movement, or you feel pain when doing so, see a physiotherapist to highlight specific areas you can improve on.