Message-ID: <56F0AE7C.9040204@hpe.com>
Date: Mon, 21 Mar 2016 22:31:24 -0400
From: Waiman Long <waiman.long@hpe.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12
MIME-Version: 1.0
To: Jan Kara <jack@suse.cz>
CC: Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
        Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux-foundation.org>,
        <linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Andi Kleen <andi@firstfloor.org>, Dave Chinner <dchinner@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [PATCH v6 1/4] lib/percpu-list: Per-cpu list with associated
 per-cpu locks
References: <1458330244-46434-1-git-send-email-Waiman.Long@hpe.com> <1458330244-46434-2-git-send-email-Waiman.Long@hpe.com> <20160321094918.GG30819@quack.suse.cz>
In-Reply-To: <20160321094918.GG30819@quack.suse.cz>
Content-Type: text/plain; charset="ISO-8859-1"; format=flowed
Content-Transfer-Encoding: 7bit
SpamDiagnosticOutput: 1:23
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Mar 2016 02:31:32.2692 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DF4PR84MB0315
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1331
Lines: 37

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