On Sat, Jul 05, 2025 at 01:23:27PM -0400, Joel Fernandes wrote: > Recently while revising RCU's cpu online checks, there was some discussion > around how IPIs synchronize with hotplug. > > Add comments explaining how preemption disable creates mutual exclusion with > CPU hotplug's stop_machine mechanism. The key insight is that stop_machine() > atomically updates CPU masks and flushes IPIs with interrupts disabled, and > cannot proceed while any CPU (including the IPI sender) has preemption > disabled.
I'm very conflicted on this. While the added comments aren't wrong, they're not quite accurate either. Stop_machine doesn't wait for people to enable preemption as such. Fundamentally there seems to be a misconception around what stop machine is and how it works, and I don't feel these comments make things better. Basically, stop-machine (and stop_one_cpu(), stop_two_cpus()) use the stopper task, a task running at the ultimate priority; if it is runnable, it will run. Stop-machine simply wakes all the stopper tasks and co-ordinates them to literally stop the machine. All CPUs have the stopper task scheduled and then they go sit in a spin-loop driven state machine with IRQs disabled. There really isn't anything magical about any of this.
