Re: [PATCH 1/3] drm/ttm: fix ttm_bo_bulk_move_helper

["Christian König" <christian.koenig-5C7GfCeVMHo@public.gmane.org>]

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@lists.freedesktop.org> on behalf of Christian König <christian.koenig@amd.com>
>>      Sent: Thursday, September 6, 2018 6:06 PM
>>      To: Huang, Ray
>>      Cc: Michel Dänzer; amd-gfx@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@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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>>
>>
>>
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