[PATCH 1/3] drm/ttm: fix ttm_bo_bulk_move_helper
Christian König
christian.koenig at amd.com
Sat Sep 8 10:43:55 UTC 2018
Am 08.09.2018 um 09:54 schrieb Huang Rui:
> On Fri, Sep 07, 2018 at 07:18:59PM +0800, Christian K�nig wrote:
>> Am 07.09.2018 um 13:02 schrieb Huang, Ray:
>>
>> Yes, that was one problem. Another was that the cutting code was buggy
>> and determined one of the positions to cut at the wrong time.
>>
>> I went through again about the list cutting behavior and wrote down with the attached picture.
>> After do the second list_cut_position, the list2 should be point the end of "before" list. And list2 won't be used anymore after list cutting. May I know am I something missed?
>>
>>
>> Let's take a look at the original code:
>>
>> list1 = is_swap ? &pos->last->swap : &pos->last->lru;
>> list2 = is_swap ? pos->first->swap.prev : pos->first->lru.prev;
>>
>> list_cut_position(&entries, lru, list1);
>> list_cut_position(&before, &entries, list2);
>>
>>
>> As far as I can see the problem is that the first list_cur_position could
>> modify the value of pos->first->lru.prev and so make the second
>> list_cut_position work on the wrong position.
>>
> I think I understood. In this case (the first element of LRU ==
> pos->first->lru), please see the picture. If we store first->lru.prev as
> list2(LRU head), after do list cutting, the first->lru.prev will overwrite
> as new head (entries), however, the orignal list2 will still point previous
> head (that is the wrong position now). We actually expected to use the
> latest first->lru.prev as the second cutting position. So we should adjust
> code sequence like below:
>
> list1 = is_swap ? &pos->last->swap : &pos->last->lru;
> list_cut_position(&entries, lru, list1);
>
> list2 = is_swap ? pos->first->swap.prev : pos->first->lru.prev;
> list_cut_position(&before, &entries, list2);
>
> Am I understanding right?
Yes, exactly. And the fix looks like what I did as well. Just only used
one variable and added some extra BUG_ON().
Christian.
>
> Thanks,
> Ray
>
>> Regards,
>> Christian.
>>
>>
>>
>> Thanks,
>> Ray
>>
>> From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> on behalf of Christian König <christian.koenig at amd.com>
>> Sent: Thursday, September 6, 2018 6:06 PM
>> To: Huang, Ray
>> Cc: Michel Dänzer; amd-gfx at lists.freedesktop.org
>> Subject: Re: [PATCH 1/3] drm/ttm: fix ttm_bo_bulk_move_helper
>>
>>
>> Am 06.09.2018 um 12:02 schrieb Huang Rui:
>>
>> On Fri, Aug 31, 2018 at 05:17:33PM +0200, Christian König wrote:
>>
>> Am 31.08.2018 um 17:15 schrieb Michel Dänzer:
>>
>> On 2018-08-31 3:10 p.m., Christian König wrote:
>>
>> Staring at the function for six hours, just to essentially move one line
>> of code.
>>
>> That sucks, but the commit log should describe what the problem was and
>> how this patch solves it.
>>
>>
>>
>> Signed-off-by: Christian König <christian.koenig at amd.com>
>> ---
>> drivers/gpu/drm/ttm/ttm_bo.c | 13 ++++++++-----
>> 1 file changed, 8 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>> index 35d53d81f486..138c98902033 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>> @@ -250,15 +250,18 @@ EXPORT_SYMBOL(ttm_bo_move_to_lru_tail);
>> static void ttm_bo_bulk_move_helper(struct ttm_lru_bulk_move_pos *pos,
>> struct list_head *lru, bool is_swap)
>> {
>> + struct list_head *list;
>> LIST_HEAD(entries);
>> LIST_HEAD(before);
>> - struct list_head *list1, *list2;
>> - list1 = is_swap ? &pos->last->swap : &pos->last->lru;
>> - list2 = is_swap ? pos->first->swap.prev : pos->first->lru.prev;
>> + reservation_object_assert_held(pos->last->resv);
>> + list = is_swap ? &pos->last->swap : &pos->last->lru;
>> + list_cut_position(&entries, lru, list);
>> +
>> + reservation_object_assert_held(pos->first->resv);
>> + list = is_swap ? pos->first->swap.prev : pos->first->lru.prev;
>> + list_cut_position(&before, &entries, list);
>>
>> So the problem was that the first list_cut_position call could result in
>> list2 pointing to la-la-land? Good catch!
>>
>> Yes, exactly. Thought that would be obvious, but going to add that
>> to the commit log.
>>
>> Can I get a tested-by? You where much better at reproducing that than I'm.
>>
>>
>> Michel, Christian, thanks so much to take care of this when I was on
>> vacation. And sorry to let you take a long time for finding the cause.
>>
>> Is that because I didn't hold the resveration before cut the list from
>> position "first" and "last"?
>>
>> Yes, that was one problem. Another was that the cutting code was buggy
>> and determined one of the positions to cut at the wrong time.
>>
>>
>> May I know in which cases, we must hold the
>> bo's reservation firstly?
>>
>> BOs are reserved to prevent moving them. E.g. when the BO isn't reserved
>> it can move around and so the LRU where you want to add/remove it could
>> change.
>>
>> Christian.
>>
>>
>> Thanks,
>> Ray
>>
>>
>>
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