[QEMU/FUSE] Discussion on Proper Termination and Async Cancellation in fuse-over-io_uring

[Brian Song <hibriansong@gmail.com>]

Hi Bernd,

We are currently working on implementing termination support for 
fuse-over-io_uring in QEMU, and right now we are focusing on how to 
clean up in-flight SQEs properly. Our main question is about how well 
the kernel supports robust cancellation for these fuse-over-io_uring 
SQEs. Does it actually implement cancellation beyond destroying the 
io_uring queue?

In QEMU FUSE export, we need a way to quickly and cleanly detach from 
the event loop and cancel any pending SQEs when an export is no longer 
in use. Ideally, we want to avoid the more drastic measure of having to 
close the entire /dev/fuse fd just to gracefully terminate outstanding 
operations.

We are not sure if there's an existing code path that supports async 
cancel for these in-flight SQEs in the fuse-over-io_uring setup, or if 
additional callbacks might be needed to fully integrate with the 
kernel's async cancel mechanism. We also realized libfuse manages 
shutdowns differently, typically by signaling a thread via eventfd 
rather than relying on async cancel.

Would love to hear your thoughts or suggestions on this!

Thanks,
Brian