Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design ofPer-CPU Reader-Writer Locks

From: Srivatsa S. Bhat
Date: Tue Feb 26 2013 - 10:20:18 EST

On 02/26/2013 07:04 PM, Lai Jiangshan wrote:
> On Tue, Feb 26, 2013 at 3:26 AM, Srivatsa S. Bhat
> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>> Hi Lai,
>> On 02/25/2013 09:23 PM, Lai Jiangshan wrote:
>>> Hi, Srivatsa,
>>> The target of the whole patchset is nice for me.
>> Cool! Thanks :-)
>>> A question: How did you find out the such usages of
>>> "preempt_disable()" and convert them? did all are converted?
>> Well, I scanned through the source tree for usages which implicitly
>> disabled CPU offline and converted them over.
> How do you scan? could you show the way you scan the source tree.
> I can follow your instructions for double checking.

Its nothing special. I grepped the source tree for anything dealing with
cpu_online_mask or its derivatives and also for functions/constructs that
rely on the cpumasks internally (eg: smp_call_function). Then I audited all
such call-sites and converted them (if needed) accordingly.

>> Its not limited to uses
>> of preempt_disable() alone - even spin_locks, rwlocks, local_irq_disable()
>> etc also help disable CPU offline. So I tried to dig out all such uses
>> and converted them. However, since the merge window is open, a lot of
>> new code is flowing into the tree. So I'll have to rescan the tree to
>> see if there are any more places to convert.
> I remember some code has such assumption:
> preempt_disable() (or something else)
> //the code assume that the cpu_online_map can't be changed.
> preempt_enable()
> It is very hard to find out all such kinds of assumptions and fixes them.
> (I notice your code mainly fixes code around send_xxxx())

The conversion can be carried out using the method I mentioned above.

Srivatsa S. Bhat

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