Messages in this thread | | | Subject | Re: [PATCH] sched: sched_clock() clocksource handling. | From | Peter Zijlstra <> | Date | Tue, 02 Jun 2009 10:00:03 +0200 |
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On Tue, 2009-06-02 at 16:54 +0900, Paul Mundt wrote: > On Tue, Jun 02, 2009 at 09:41:35AM +0200, Peter Zijlstra wrote: > > On Tue, 2009-06-02 at 16:35 +0900, Paul Mundt wrote: > > > > > > We already do via select_clocksource(), if we are unregistering the > > > current one then a new one with the flag set is selected. Before that, > > > the override is likewise given preference, and we fall back on jiffies if > > > there is nothing else. I suppose we could try and find the "best" one, > > > but I think the override and manual clocksource selection should be fine > > > for this. > > > > Ah, ok. So unregister calls select_clocksource again? That does leave us > > a small window with jiffies, but I guess that's ok. > > > A synchronize_rcu() would fix that up, but I think a small window with > jiffies is less painful than sorting out RCU ordering and synchronization > for a corner case of a corner case ;-) > > > > Now that you mention it though, the sched_clocksource() assignment within > > > select_clocksource() happens underneath the clocksource_lock, but is not > > > using rcu_assign_pointer(). > > > > Right, that would want fixing indeed. > > > > > If the assignment there needs to use > > > rcu_assign_pointer() then presumably all of the unlock paths that do > > > select_clocksource() will have to synchronize_rcu()? > > > > No, you only have to do sync_rcu() when stuff that could have referenced > > is going away and you cannot use call_rcu(). > > > > So when selecting a new clocksource, you don't need synchonization > > because stuff doesn't go away (I think :-) > > Ok, that keeps things more simplified then. How does this look?
Looks fine to me,
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
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