Messages in this thread | | | Date | Tue, 2 Jun 2009 22:17:23 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] sched: sched_clock() clocksource handling. |
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On Tue, 2 Jun 2009, 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. > > > 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 :-)
Hmm, no. In the unregister case stuff _IS_ going away. That's why you unregister in the first place, right ?
Thanks,
tglx
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