Messages in this thread | | | Date | Fri, 7 Nov 2008 11:51:12 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb() |
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* David Howells (dhowells@redhat.com) wrote: > Nicolas Pitre <nico@cam.org> 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.
Mathieu
> > > /* > > > * Caller must provide locking to protect *caller_state > > > */ > > > > NO! This is meant to be LOCK FREE! > > Absolutely. > > David
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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