To inject some numbers to this claim, and unless I am badly mistaken, the failure chance is 99.999999%. Raising that number to the power of approximately 70 million yields around 50 % success probability. It is fundamentally a matter of chance, so 100% success can never be achieved, though something very close to it can be achieved, of course.
In any case this sort of probabilities require means to fire the attack several times per second or it will probably take years of continuous attempting before succeeding. Unfortunately ptrace sounds like the sort of thing you can try thousands of times per second.