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Subject: Re: [PATCH -v3 6/8] fsnotify: add group priorities
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Eric Paris <eparis@redhat.com>
Cc: linux-kernel@vger.kernel.org, malware-list@lists.printk.net,
       viro@zeniv.linux.org.uk, akpm@linux-foundation.org,
       alan@lxorguk.ukuu.org.uk, arjan@infradead.org, hch@infradead.org
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Date: Thu, 27 Nov 2008 17:25:09 +0100
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On Tue, 2008-11-25 at 12:21 -0500, Eric Paris wrote:
> In preperation for blocking fsnotify calls group priorities must be added.
> When multiple groups request the same event type the lowest priority group
> will receive the notification first.

> @@ -114,9 +117,26 @@ struct fsnotify_group *fsnotify_find_group(unsigned int group_num, unsigned long
>  
>  	group->ops = ops;
>  
> -	/* add it */
> -	list_add_rcu(&group->group_list, &fsnotify_groups);
> +	/* Do we need to be the first entry? */
> +	if (list_empty(&fsnotify_groups)) {
> +		list_add_rcu(&group->group_list, &fsnotify_groups);
> +		goto out;
> +	}
> +
> +	list_for_each_entry(group_iter, &fsnotify_groups, group_list) {
> +		/* insert in front of this one? */
> +		if (priority < group_iter->priority) {
> +			/* I used list_add_tail() to insert in front of group_iter...  */
> +			list_add_tail_rcu(&group->group_list, &group_iter->group_list);
> +			break;
> +		}
>  
> +		/* are we at the end?  if so insert at end */
> +		if (list_is_last(&group_iter->group_list, &fsnotify_groups)) {
> +			list_add_tail_rcu(&group->group_list, &fsnotify_groups);
> +			break;
> +		}
> +	}
>  out:
>  	mutex_unlock(&fsnotify_grp_mutex);
>  	fsnotify_recalc_global_mask();

What priority range do you need to cater for, and how many groups? I can
imagine for many groups and limit range a priority list might be better
suited.