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

[Huang Rui <ray.huang-5C7GfCeVMHo@public.gmane.org>]

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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?

Thanks,
Ray

> Regards,
> Christian.
> 
> 
> 
>     Thanks,
>     Ray
> 
>     From: amd-gfx <amd-gfx-bounces-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org> on behalf of Christian König <christian.koenig-5C7GfCeVMHo@public.gmane.org>
>     Sent: Thursday, September 6, 2018 6:06 PM
>     To: Huang, Ray
>     Cc: Michel Dänzer; amd-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.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-5C7GfCeVMHo@public.gmane.org>
>                     ---
>                        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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