The only way this could be considered a issue is that it appears that the Linux kernel added the rqspinlock which is supposed to automatically detect incorrect code at runtime and kind of “un-incorrect” it. That piece of code did not correctly detect callers who were blindly using it incorrectly in ways that the writers probably expected to detect.

However, this entire escapade is absurd. Not only does this indicate that eBPF has gotten extensions that grossly violate any concept of sandboxing that proponents claim, I do not see how you can effectively program in the rqspinlock environment. Any lock acquire can now fail with a timeout because some poorly written eBPF program decided that deadlocks were a enjoyable activity. Every single code path that acquires more than one lock must be able to guarantee global consistency before every lock acquire.

For instance, you can not lock a sub-component for modification and then acquire a whole component lock to rectify the state since that second lock acquire may arbitrarily fail.

Furthermore, even if you do that all it does is turn deadlocks due to incorrect code into incredibly long multi-millisecond denials of service due to incorrect code. I mean, yes, bad is better than horrible, but it is still bad.

It is very confusing how Linux source code has macros with names that make them look like functions. At first view it looks like "flags" is passed uninitialized, but it's a temporary save variable used by a macro. Sigh.

The minimized repro seems like something many other eBPF programs will do. This makes me wonder why such kernel issues weren’t found earlier. Is this code utilizing some new eBPF capabilities in recent kernels?

TL;DR the main issue arises because the context switch and sampling event both need to be written to the `ringBuffer` eBPF map. sampling event lock needs to be taken in an NMI which is by definition non-maskable. This leads to lock contention and recursive locks etc as explained when context switch handler tries to do the same thing.

Why not have context switches write to ringBuffer1 and sampling events write to ringBuffer2 (i.e. use different ringBuffers). This way buggy kernels should work properly too !?