FUSE_INTERRUPT — fuse_interrupt_in (8 bytes). Carries the unique of a previously-issued in-flight request that the kernel wants to cancel (e.g. because userspace received a signal during a blocking SETLKW).

INTERRUPT itself has no reply on the wire — the original request is what the kernel waits for, typically with EINTR. Handlers that don't recognise the target unique should silently drop the message (the original may have already replied or might never have been blockable).