linux-kernel - Re: [PATCH vfs 1/2] lib: implement ptrset

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Message-Id: <20141113142333.39fc29592019a397131fb03c@linux-foundation.org>
Date:	Thu, 13 Nov 2014 14:23:33 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH vfs 1/2] lib: implement ptrset

On Thu, 13 Nov 2014 17:09:27 -0500 Tejun Heo <tj@...nel.org> wrote:

> Implement set of pointers.  Pointers can be added, deleted and
> iterated.  It's currently implemented as a thin rbtree wrapper making
> addition and removal O(log N).  A drawback is that iteration isn't RCU
> safe, which is okay for now.  This will be used to remove
> inode->i_devices.
> 

Confused.

>
> --- /dev/null
> +++ b/include/linux/ptrset.h
> @@ -0,0 +1,74 @@
> +/*
> + * include/linux/ptrset.h - set of pointers
> + *
> + * Copyright (C) 2014		Tejun Heo, Red Hat Inc.
> + *
> + * A ptrset contains an unordered set of pointers.  Pointers can be added,
> + * deleted and iterated.  Addition and removal complexities are O(log N)
> + * where N is the total number of elements in the ptrset.
> + */
> +
> +#ifndef __PTRSET_H
> +#define __PTRSET_H
> +
> +#include <linux/preempt.h>
> +#include <linux/rbtree.h>
> +
> +struct ptrset {
> +	struct rb_root		root;
> +};
> +
> +struct ptrset_elem {
> +	void			*ptr;
> +	struct rb_node		node;
> +};
> +
> +struct ptrset_iter {
> +	struct ptrset_elem	*pos;
> +	struct ptrset_elem	*next;
> +};

This seems rather slow and bloaty.  Why not

struct tjpointer {
	struct list_head list;
	void *pointer;
};

And then callers do things like

	struct tjpointer *tjp;

	lock();

	for_each_tjpointer(tjp, &my_tjpointer_list) {
		foo(tjp->ptr);
	}

	tjpointer_del(tjp);

	unlock();

That's less storage, vastly less support code, insertion and removal
are O(1) and it doesn't need the ghastly preload thing.


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