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

[Hillf Danton]

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.

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