Messages in this thread | | | Date | Fri, 7 Dec 2012 10:31:41 -0800 | From | Tejun Heo <> | Subject | Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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Hello, Srivatsa.
On Fri, Dec 07, 2012 at 11:54:01PM +0530, Srivatsa S. Bhat wrote: > > lg_lock doesn't do local nesting and I'm not sure how big a deal that > > is as I don't know how many should be converted. But if nesting is an > > absolute necessity, it would be much better to implement generic > > rwlock variant (say, lg_rwlock) rather than implementing unusual > > cpuhotplug-specific percpu synchronization construct. > > To be honest, at a certain point in time while designing this, I did > realize that this was getting kinda overly complicated ;-) ... but I > wanted to see how this would actually work out when finished and get > some feedback on the same, hence I posted it out. But this also proves > that we _can_ actually compete with the flexibility of preempt_disable() > and still be safe with respect to locking, if we really want to ;-)
I got confused by comparison to preempt_disable() but you're right that percpu rwlock shouldn't be able to introduce locking dependency which doesn't exist with non-percpu rwlock. ie. write locking should be atomic w.r.t. to all readers. At the simplest, this can be implemented by writer backing out all the way if try-locking any CPU fails and retrying the whole thing. That should be correct but has the potential of starving the writer.
What we need here is a generic percpu-rwlock. I don't know which exact implementation strategy we should choose. Maybe your switching to global rwlock is the right solution. But, at any rate, I think it would be best to implement proper percpu-rwlock and then apply it to CPU hotplug. It's actually gonna be pretty fitting as get_online_cpus() is being converted to percpu-rwsem. IIUC, Oleg has been working on this for a while now. Oleg, what do you think?
Thanks.
-- tejun
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