Oh of course - interrupts. Right. Which is what [I now understand] preeemptible means; system interrupts can intercept normal execution and cause the kernel to go do something else for a minute.

Thanks so much for explaining the read-write conditions too, I now understand this perfectly.

Thanks for the recommendation about the LWN subscription, I'll file that away as a serious consideration. For the meantime, I'm curious - does the subscriber-only content go free after a certain period, or do certain articles remain subscriber-only?