linux-kernel - Re: [PATCH v3 1/4] lib/dlock-list: Distributed and lock-protected lists

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Date:	Wed, 20 Jul 2016 18:02:54 -0400
From:	Waiman Long <waiman.long@....com>
To:	Tejun Heo <tj@...nel.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v3 1/4] lib/dlock-list: Distributed and lock-protected
 lists

On 07/19/2016 03:23 PM, Tejun Heo wrote:
> Hello,
>
> On Tue, Jul 19, 2016 at 02:42:31PM -0400, Waiman Long wrote:
>>>> +int alloc_dlock_list_head(struct dlock_list_head *dlist)
>>>> +{
>>>> +	struct dlock_list_head dlist_tmp;
>>>> +	int cpu;
>>>> +
>>>> +	dlist_tmp.head = alloc_percpu(struct dlock_list_head_percpu);
>>>> +	if (!dlist_tmp.head)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	for_each_possible_cpu(cpu) {
>>>> +		struct dlock_list_head_percpu *head;
>>>> +
>>>> +		head = per_cpu_ptr(dlist_tmp.head, cpu);
>>>> +		INIT_LIST_HEAD(&head->list);
>>>> +		head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
>>>> +		lockdep_set_class(&head->lock,&dlock_list_key);
>>>> +	}
>>>> +
>>>> +	dlist->head = dlist_tmp.head;
>>> Just use dlist->head directly or use local __perpcu head pointer?
>> I just don't want to expose the structure to world until it is fully
>> initialized. If you think I am over-cautious, I can use dlist->head as
>> suggested.
> I don't think it makes any actual difference.  No strong opinion
> either way.  Just use local __percpu head pointer then?

I have run sparse on dlock_list.c. There is no need to use the __percpu 
tag here. The head gets assigned the result of per_cpu_ptr() which has 
no __percpu annotation. I actually got sparse warning if I used the 
__percpu tag.

Cheers,
Longman