In the top part of the picture, its that a hole in the back of your seattube that the crack is coming from? Is that an original feature of your frame if so? Also, i wouldn't ride it. If there's visible damage, there is almost certainly invisible damage and that could get really bad really quickly. Maybe indoor where the potential for catastrophe is limited, definitely not outside.

Keep riding what could possibly go wrong?

 
I have seen where people have drilled a hole at each end of the crack to stop the spreading of the crack. Not sure if that is what I would though.

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Once, I was fast. But I got over it.

I sure as hell wouldn't ride a bike with a 10 cm long crack. But hey, it's your safety that's on the line, not mine. That particular spot is probably less likely to have a sudden catastrophic failure than some other spots on the bike but there's no way to be sure. Hopefully if it fails further during a ride you'll hear or feel something and you either have to call for a ride or ride out of the saddle until you get home. Worst case scenario it fails suddenly, pitches you backward and you go down, hopefully not impaling your junk on the remnants of your seat tube.

I'd ride it... on a trainer.

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THIS, or get it fixed. I wouldn't risk it outside.

I cannot see what else the seat tube looks like, but it should be fixable. I had an issue with the collar that clamps the seat, but as soon as I realized what was going on, I handed it to the bike shop and got a ride home (I had ridden to the shop for a group ride when I noticed it).

I have a madone 6 that has this type of crack.. got a repair kit from amazon. 1 layer on the outside, 2 layers on the inside. Lots of sanding on the outside so the seatmast could fit again and called it done.

Carbon repair has gotten so good and so cheap that I don't know why you wouldn't get that fixed.
Andy

That crack being as long as it is, there is almost certainly a network of transverse fractures and delaminations inside the frame that you won't be able to fix by adding material onto the outside. You'd need to remove the damaged material and bond an entirely new section on to do the job right. Also, the load path depends on the layup there and in the surrounding areas.

I seriously doubt this is something you can fix yourself.

If you have to ask how to fix it you should not be attempting it yourself.
What does the inside look like?
What does it sound like?
How does it feel when you try to flex it?

From your picture it looks like it s dimensionally distorted.
Take it too a professional or junk it.

There are a lot of places now that repair frames. You can not even tell they were repaired.

I've been riding and racing a tri bike with a seat tube crack for over 4 years but it is only 1 cm and hasn't grown. I wouldn't one that long. Get it fixed.