Messages in this thread | | | Date | Sat, 08 Dec 2012 00:08:01 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
| |
On 12/08/2012 12:01 AM, Tejun Heo wrote: > 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.
Yep!
> 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. >
Exactly! This is what I mentioned yesterday in the link below, and said that its not good because of writer starvation ("too wasteful"): https://lkml.org/lkml/2012/12/6/290
> 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? >
Hmm, that sounds good.
Regards, Srivatsa S. Bhat
| |