Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offlinefrom atomic context

From: Srivatsa S. Bhat
Date: Fri Dec 07 2012 - 13:25:24 EST

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
>> 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 ;-)

Srivatsa S. Bhat

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