linux-kernel - Re: [PATCH] rbtree: introduce three helpers about sort-order iteration

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Date:   Tue, 10 Mar 2020 10:02:07 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     akpm@...ux-foundation.org, walken@...gle.com, rfontana@...hat.com,
        dbueso@...e.de, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        chenqiwu <chenqiwu@...omi.com>
Subject: Re: [PATCH] rbtree: introduce three helpers about sort-order
 iteration

On Mon, Mar 09, 2020 at 05:55:11PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 10, 2020 at 12:02:14AM +0800, qiwuchen55@...il.com wrote:
> > +/**
> > + * rbtree_for_each_entry_safe - iterate in sort-order over of given type
> > + * allowing the backing memory of @pos to be invalidated.
> > + * @pos:	the 'type *' to use as a loop cursor.
> > + * @n:		another 'type *' to use as temporary storage.
> > + * @root:	'rb_root *' of the rbtree.
> > + * @field:	the name of the rb_node field within 'type'.
> > + *
> > + * rbtree_order_for_each_entry_safe() provides a similar guarantee as
> > + * list_for_each_entry_safe() and allows the sort-order iteration to
> > + * continue independent of changes to @pos by the body of the loop.
> > + *
> > + * Note, however, that it cannot handle other modifications that re-order the
> > + * rbtree it is iterating over. This includes calling rb_erase() on @pos, as
> > + * rb_erase() may rebalance the tree, causing us to miss some nodes.
> > + */
> > +#define rbtree_for_each_entry_safe(pos, n, root, field) \
> > +	for (pos = rb_entry_safe(rb_first(root), typeof(*pos), field); \
> > +	     pos && ({ n = rb_entry_safe(rb_next(&pos->field), \
> > +			typeof(*pos), field); 1; }); \
> > +	     pos = n)
> 
> Since this cannot deal with rb_erase(), what exactly is the purpose of
> this thing?

It's just a copy of rbtree_postorder_for_each_entry_safe() for
the usage scenario of sort-order iteration. It can be used for
walking the tree and free all entries of the given type.