On 09.09.25 16:05, Peter Zijlstra wrote:
> On Tue, Sep 09, 2025 at 02:04:30PM +0200, Christian König wrote:
>> Hi Arun,
>>
>> On 09.09.25 11:56, Arunpravin Paneer Selvam wrote:
>> [SNIP]
>>
>>> +/**
>>> + * rbtree_for_each_entry_safe - iterate in-order over rb_root safe against
>>> removal
>>> + *
>>> + * @pos: the 'type *' to use as a loop cursor
>>> + * @n: another 'type *' to use as temporary storage
>>> + * @root: 'rb_root *' of the rbtree
>>> + * @member: the name of the rb_node field within 'type'
>>> + */
>>> +#define rbtree_for_each_entry_safe(pos, n, root, member) \
>>> + for ((pos) = rb_entry_safe(rb_first(root), typeof(*(pos)), member), \
>>> + (n) = (pos) ? rb_entry_safe(rb_next(&(pos)->member),
>>> typeof(*(pos)), member) : NULL; \
>>> + (pos); \
>>> + (pos) = (n), \
>>> + (n) = (pos) ? rb_entry_safe(rb_next(&(pos)->member),
>>> typeof(*(pos)), member) : NULL)
>>
>> As far as I know exactly that operation does not work on an R/B tree.
>>
>> See the _safe() variants of the for_each_ macros are usually used to iterate
>> over a container while being able to remove entries.
>>
>> But because of the potential re-balance storing just the next entry is not
>> sufficient for an R/B tree to do that as far as I know.
>>
>> Please explain how exactly you want to use this macro.
>
> So I don't much like these iterators; I've said so before. Either we
> should introduce a properly threaded rb-tree (where the NULL child
> pointers encode a linked list), or simply keep a list_head next to the
> rb_node and use that.
I agree, something is clearly fishy here.
> The rb_{next,prev}() things are O(ln n), in the worst case they do a
> full traversal up the tree and a full traversal down the other branch.
Yeah from the logic that is exactly what is supposed to happen in the
__force_merge() function.
The question is rather why does that function exists in the first place? The
operation doesn't look logical to me.
For drm_buddy_reset_clear() and drm_buddy_fini() we should use
rbtree_postorder_for_each_entry_safe() instead.
And during normal allocation __force_merge() should never be used.
> That said; given 'next' will remain an existing node, only the 'pos'
> node gets removed, rb_next() will still work correctly, even in the face
> of rebalance.
Good to know!
Regards,
Christian.
