linux-kernel - Re: [PATCH] rbtree: clarify documentation of rbtree_postorder_for_each_entry

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Date:	Tue, 6 Oct 2015 13:25:55 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Cody P Schafer <dev@...yps.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	John de la Garza <john@...ev.com>,
	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rbtree: clarify documentation of
 rbtree_postorder_for_each_entry_safe()

Hello,

On Sun, Oct 04, 2015 at 06:32:50PM -0400, Cody P Schafer wrote:
> Doing this will cause random nodes to be missed by the iteration because
> rb_erase() may rebalance the tree, changing the ordering that we're
> trying to iterate over.
> 
> The previous documentation for rbtree_postorder_for_each_entry_safe()
> wasn't clear that this wasn't allowed, it was taken from the docs for
> list_for_each_entry_safe(), where erasing isn't a problem due to
> list_del() not reordering.

Ugh... that's a misleading name for an iterator if it doesn't allow
removal of elements during iteration.

> Explicitly warn developers about this potential pit-fall.
> 
> Note that I haven't fixed the actual issue that (it appears) the commit
> referenced above introduced (not familiar enough with that code).
> 
> In general (and in this case), the patterns to follow are:
>  - switch to rb_first() + rb_erase(), don't use
>    rbtree_postorder_for_each_entry_safe().

I'll update it to a while loop on rb_first().

Thanks.

-- 
tejun
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