linux-kernel - Re: [PATCH v6 1/4] lib/percpu-list: Per-cpu list with associated per-cpu locks

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Message-ID: <56F0AE7C.9040204@hpe.com>
Date:	Mon, 21 Mar 2016 22:31:24 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jan Kara <jack@...e.cz>
CC:	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.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 v6 1/4] lib/percpu-list: Per-cpu list with associated
 per-cpu locks

On 03/21/2016 05:49 AM, Jan Kara wrote:
> On Fri 18-03-16 15:44:01, Waiman Long wrote:
>> +static __always_inline bool
>> +__pcpu_list_next_cpu(struct pcpu_list_head *head, struct pcpu_list_state *state)
>> +{
>> +	if (state->lock)
>> +		spin_unlock(state->lock);
>> +next_cpu:
>> +	/*
>> +	 * for_each_possible_cpu(cpu)
>> +	 */
>> +	state->cpu = cpumask_next(state->cpu, cpu_possible_mask);
>> +	if (state->cpu>= nr_cpu_ids)
>> +		return false;	/* All the per-cpu lists iterated */
>> +
>> +	state->head =&per_cpu_ptr(head, state->cpu)->list;
>> +	if (list_empty(state->head))
>> +		goto next_cpu;
>> +
>> +	state->lock =&per_cpu_ptr(head, state->cpu)->lock;
>> +	spin_lock(state->lock);
>> +	state->curr = list_entry(state->head->next,
>> +				 struct pcpu_list_node, list);
>> +	return true;
> Waiman, I repeat it for the third time as you keep ignoring it: This is
> *racy*. The list for state->cpu can be empty by the time you acquire
> state->lock and thus state->curr will point somewhere around the head of
> the list but definitely not to a list member where it should.
>
> 								Honza

I am sorry for missing your previous comment. Yes, it is possible that 
the list is empty after the lock. So I should have checked for that 
before returning. Thanks for reminding me that. I will fix that later on.

Cheers,
Longman