Messages in this thread | | | Date | Fri, 07 Dec 2012 23:54:01 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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On 12/07/2012 11:27 PM, Tejun Heo wrote: > On Fri, Dec 07, 2012 at 11:08:13PM +0530, Srivatsa S. Bhat wrote: >> 4. No deadlock possibilities >> >> Per-cpu locking is not the way to go if we want to have relaxed rules >> for lock-ordering. Because, we can end up in circular-locking dependencies >> as explained in https://lkml.org/lkml/2012/12/6/290 >> >> So, avoid per-cpu locking schemes (per-cpu locks/per-cpu atomic counters >> with spin-on-contention etc) as much as possible. > > I really can't say I like this approach. percpu locking is very > tricky to get right and difficult to get right and we should try our > best to avoid implementing subsystem specific ones as much as > possible. Also, I think the right approach would be auditing each > get_online_cpus_atomic() callsites and figure out proper locking order > rather than implementing a construct this unusual especially as > hunting down the incorrect cases shouldn't be difficult given proper > lockdep annotation. > > 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 ;-)
Regards, Srivatsa S. Bhat
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