That doesn't really explain it though, IMO. IIUC, it's a sequence of instructions that either runs to completion atomically or doesn't. If it is interrupted by anything the kernel jumps you to the abort/retry vector you set with a guarantee that the last instruction in the sequence was not executed.

(Based on my reading of the LWN article rwmj posted).

Yes, the API contract isn't "don't interrupt me during this critical section" it's "if you have to interrupt me during this critical section, go to this recovery/restart code".

There is a time-slice extension feature in the works that's roughly "please let me finish this critical section before you interrupt me". But a hard guarantee that userspace code won't be interrupted is probably untenable in a preemptive multitasking system.