netdev - Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

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Date:	Mon, 11 Feb 2013 01:50:10 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	linux-doc@...r.kernel.org, peterz@...radead.org,
	fweisbec@...il.com, mingo@...nel.org, linux-arch@...r.kernel.org,
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	wangyun@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	rusty@...tcorp.com.au, rostedt@...dmis.org, rjw@...k.pl,
	namhyung@...nel.org, tglx@...utronix.de,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, sbw@....edu, tj@...nel.org,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design of
 Per-CPU Reader-Writer Locks

On 02/11/2013 01:43 AM, Oleg Nesterov wrote:
> On 02/11, Srivatsa S. Bhat wrote:
>>
>> On 02/09/2013 04:40 AM, Paul E. McKenney wrote:
>>>> +static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock)
>>>> +{
>>>> +	unsigned int cpu;
>>>> +
>>>> +	drop_writer_signal(pcpu_rwlock, smp_processor_id());
>>>
>>> Why do we drop ourselves twice?  More to the point, why is it important to
>>> drop ourselves first?
>>>
>>
>> I don't see where we are dropping ourselves twice. Note that we are no longer
>> in the cpu_online_mask, so the 'for' loop below won't include us. So we need
>> to manually drop ourselves. It doesn't matter whether we drop ourselves first
>> or later.
> 
> Yes, but this just reflects its usage in cpu-hotplug. cpu goes away under
> _write_lock.
> 

Ah, right. I guess the code still has remnants from the older version in which
this locking scheme wasn't generic and was tied to cpu-hotplug alone..

> Perhaps _write_lock/unlock shoud use for_each_possible_cpu() instead?
> 

Hmm, that wouldn't be too bad.

> Hmm... I think this makes sense anyway. Otherwise, in theory,
> percpu_write_lock(random_non_hotplug_lock) can race with cpu_up?
> 

Yeah, makes sense. Will change it to for_each_possible_cpu().
And I had previously fixed such races with lglocks with a complicated scheme (to
avoid the costly for_each_possible loop), which was finally rewritten to use
for_each_possible_cpu() for the sake of simplicity..
Regards,
Srivatsa S. Bhat

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