Messages in this thread | | | Date | Thu, 24 Oct 2002 13:11:03 +0530 | From | Dipankar Sarma <> | Subject | Re: [PATCH] NMI request/release, version 4 |
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On Wed, Oct 23, 2002 at 04:53:34PM -0500, Corey Minyard wrote: > >After local_irq_count() went away, the idle CPU check was broken > >and that meant that if you had an idle CPU, it could hold up RCU > >grace period completion. > > > Ah, much better. That seems to fix it.
Great! Do you have any latency numbers ? Just curious.
> > >It might just be simpler to use completions instead - > > > > call_rcu(&(handler->rcu), free_nmi_handler, handler); > > init_completion(&handler->completion); > > spin_unlock_irqrestore(&nmi_handler_lock, flags); > > wait_for_completion(&handler->completion); > > > >and do > > > > complete(&handler->completion); > > > >in the the RCU callback. > > > I was working under the assumption that the spinlocks were needed. But > now I see that there are spinlocks in wait_for_completion. You did get > init_completion() and call_rcu() backwards, they would need to be the > opposite order, I think.
AFAICS, the ordering of call_rcu() and init_completion should not matter because the CPU that is executing them would not have gone through a quiescent state and thus the RCU callback cannot happen. Only after a context swtich in wait_for_completion(), the callback is possible.
> > >Also, now I think your original idea of "Don't do this!" :) was right. > >Waiting until an nmi handler is seen unlinked could make a task > >block for a long time if another task re-installs it. You should > >probably just fail installation of a busy handler (checked under > >lock). > > > Since just about all of these will be in modules at unload time, I'm > thinking that the way it is now is better. Otherwise, someone will mess > it up. IMHO, it's much more likely that someone doesn't handle the > callback correctly than someone reused the value before the call that > releases it returns. I'd prefer to leave it the way it is now.
Oh, I didn't mean the part about waiting in release_nmi() until the release is complete (RCU callback done). That is absolutely necessary. I was talking about looping until the handler is not busy any more. I think it is safe to just do a wait_for_completion() and return in release_nmi().
Thanks Dipankar
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