linux-kernel - Re: [RFC 5/6] mm, cpuset: always use seqlock when changing task's nodemask

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Date:   Wed, 12 Apr 2017 10:18:21 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        'Li Zefan' <lizefan@...wei.com>,
        'Michal Hocko' <mhocko@...nel.org>,
        'Mel Gorman' <mgorman@...hsingularity.net>,
        'David Rientjes' <rientjes@...gle.com>,
        'Christoph Lameter' <cl@...ux.com>,
        'Hugh Dickins' <hughd@...gle.com>,
        'Andrea Arcangeli' <aarcange@...hat.com>,
        'Anshuman Khandual' <khandual@...ux.vnet.ibm.com>,
        "'Kirill A. Shutemov'" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC 5/6] mm, cpuset: always use seqlock when changing task's
 nodemask

On 04/12/2017 10:10 AM, Hillf Danton wrote:
> On April 11, 2017 10:06 PM Vlastimil Babka wrote: 
>>
>>  static void cpuset_change_task_nodemask(struct task_struct *tsk,
>>  					nodemask_t *newmems)
>>  {
>> -	bool need_loop;
>> -
>>  	task_lock(tsk);
>> -	/*
>> -	 * Determine if a loop is necessary if another thread is doing
>> -	 * read_mems_allowed_begin().  If at least one node remains unchanged and
>> -	 * tsk does not have a mempolicy, then an empty nodemask will not be
>> -	 * possible when mems_allowed is larger than a word.
>> -	 */
>> -	need_loop = task_has_mempolicy(tsk) ||
>> -			!nodes_intersects(*newmems, tsk->mems_allowed);
>>
>> -	if (need_loop) {
>> -		local_irq_disable();
>> -		write_seqcount_begin(&tsk->mems_allowed_seq);
>> -	}
>> +	local_irq_disable();
>> +	write_seqcount_begin(&tsk->mems_allowed_seq);
>>
>> -	nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
>>  	mpol_rebind_task(tsk, newmems);
>>  	tsk->mems_allowed = *newmems;
>>
>> -	if (need_loop) {
>> -		write_seqcount_end(&tsk->mems_allowed_seq);
>> -		local_irq_enable();
>> -	}
>> +	write_seqcount_end(&tsk->mems_allowed_seq);
>>
> Doubt if we'd listen irq again.

Ugh, thanks for catching this. Looks like my testing config didn't have
lockup detectors enabled.

>>  	task_unlock(tsk);
>>  }
>> --
>> 2.12.2
>