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?  

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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