The bisect pointed at the right patch series which is already very nice.
If the bisect script fails because of another problem, it is often possible to refine the script, or run bisect again on a fixed up branch.
If the bisect script fails because of another problem, it is often possible to refine the script, or run bisect again on a fixed up branch.
1) it's a kernel bug; there is a regression in the kernel's documented behaviour
2) it's a bug in your tool because it took advantage of undefined or undocumented behaviour
The bisect found exactly the right patch series, but bisect can only answer the question 'when did it stop working', not 'why'.
You can read about many instances of 2) on Raymond Chen's blog The Old New Thing.