On Fri, 7 Nov 2008, Mathieu Desnoyers wrote: > * David Howells (dhowells@xxxxxxxxxx) wrote: > > Nicolas Pitre <nico@xxxxxxx> wrote: > > > > > > I mean, the darned thing is called from sched_clock(), which can be > > > > concurrently called on separate CPUs and which can be called from > > > > interrupt context (with an arbitrary nesting level!) while it was running > > > > in process context. > > > > > > Yes! And this is so on *purpose*. Please take some time to read the > > > comment that goes along with it, and if you're still not convinced then > > > look for those explanation emails I've already posted. > > > > I agree with Nicolas on this. It's abominably clever, but I think he's right. > > > > The one place I remain unconvinced is over the issue of preemption of a process > > that is in the middle of cnt32_to_63(), where if the preempted process is > > asleep for long enough, I think it can wind time backwards when it resumes, but > > that's not a problem for the one place I want to use it (sched_clock()) because > > that is (almost) always called with preemption disabled in one way or another. > > > > The one place it isn't is a debugging case that I'm not too worried about. > > > > I am also concerned about the non-preemption off case. > > Then I think the function should document that it must be called with > preempt disabled. I explained several times already why I disagree. Preemption is not a problem unless you're preempted away for long enough, or IOW if your counter is too fast. And no, ^Z on a process doesn't create preemption. This is a signal that gets acted upon far away from the middle of cnt32_to_63(). Nicolas -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
