Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available
From: Christian KÃnig
Date: Mon Mar 05 2018 - 06:51:14 EST
Well, not really.
It's just that reservation_object_reserve_shared() is called multiple
times without actually adding fences.
That is a perfectly normal use case, so nothing special here.
Christian.
Am 05.03.2018 um 12:47 schrieb Liu, Monk:
Can you give more details ? thanks
/Monk
-----Original Message-----
From: Koenig, Christian
Sent: 2018å3æ5æ 19:39
To: Liu, Monk <Monk.Liu@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available
Am 05.03.2018 um 12:37 schrieb Liu, Monk:
But the thing confuse me is according to the design, if driver keep
calling reserve_shared() prior to add_fence(), and with lock held of cause, That BUG() shouldn't hit, so there are two things in face looks weired to me:
1) by design in reserve_shared(), obj->staged should be already NULL,
so why we kfree on it
No, that is not correct.
2) in fact, amdgpu can hit the case that obj->staged is not NULL in
reserved_shared(), don't know how it lead here
We reserved a fence slot without using it, so it is still there when
reserve_shared() is called.
Christian.
Any thought ?
/Monk
-----Original Message-----
From: Koenig, Christian
Sent: 2018å3æ5æ 19:29
To: Liu, Monk <Monk.Liu@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx;
linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when
slot available
Am 05.03.2018 um 12:25 schrieb Liu, Monk:
And by the way, I add "if (staged!=NULL) BUG();" prior to
"kfree(obj->staged)" in reserve_shared() routine, and this BUG() is actually hit, The stack dump shows it is hit during the vm_bo_update() in gem_va_update()...
That is expected. The staged handling just makes sure that there is room available, it doesn't guarantee that it is actually used.
E.g. we can end up reserving a fence slot, but then find that we actually don't need it.
Christian.
Besides, the whole reservation logic still looks a little weired to me ... especially this staged part ...
Thanks
/Monk
-----Original Message-----
From: Christian KÃnig [mailto:ckoenig.leichtzumerken@xxxxxxxxx]
Sent: 2018å3æ5æ 19:22
To: Liu, Monk <Monk.Liu@xxxxxxx>; Koenig, Christian
<Christian.Koenig@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx;
linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when
slot available
Am 05.03.2018 um 08:55 schrieb Liu, Monk:
Hi Christian
You are right on that part of obj-staged is set to NULL in
add_fence, So my following question will be why we kfree(obj->staged) in reserve_shared() if staged is always NULL in that point ?
Good question, I haven't wrote code that so I can't fully answer.
Maybe Chris or Maarten know more about that.
Christian.
Thanks
/Monk
-----Original Message-----
From: Christian KÃnig [mailto:ckoenig.leichtzumerken@xxxxxxxxx]
Sent: 2018å2æ28æ 16:27
To: Liu, Monk <Monk.Liu@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx;
linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged
when slot available
Am 28.02.2018 um 07:44 schrieb Monk Liu:
under below scenario the obj->fence would refer to a wild pointer:
1,call reservation_object_reserved_shared
2,call reservation_object_add_shared_fence
3,call reservation_object_reserved_shared
4,call reservation_object_add_shared_fence
in step 1, staged is allocated,
in step 2, code path will go
reservation_object_add_shared_replace()
and obj->fence would be assigned as staged (through
RCU_INIT_POINTER)
in step 3, obj->staged will be freed(by simple kfree), which make
obj->fence point to a wild pointer...
Well that explanation is still nonsense. See
reservation_object_add_shared_fence:
ÂÂÂÂÂÂÂ obj->staged = NULL;
Among the first things reservation_object_add_shared_fence() does is
it sets obj->staged to NULL.
So step 3 will not free anything and we never have a wild pointer.
Regards,
Christian.
in step 4, code path will go
reservation_object_add_shared_inplace()
and inside it the @fobj (which equals to @obj->staged, set by above
steps) is already a wild pointer
should remov the kfree on staged in
reservation_object_reserve_shared()
Change-Id: If7c01f1b4be3d3d8a81efa90216841f79ab1fc1c
Signed-off-by: Monk Liu <Monk.Liu@xxxxxxx>
---
drivers/dma-buf/reservation.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/drivers/dma-buf/reservation.c
b/drivers/dma-buf/reservation.c index 375de41..b473ccc 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -74,12 +74,9 @@ int reservation_object_reserve_shared(struct reservation_object *obj)
old = reservation_object_get_list(obj);
if (old && old->shared_max) {
- if (old->shared_count < old->shared_max) {
- /* perform an in-place update */
- kfree(obj->staged);
- obj->staged = NULL;
+ if (old->shared_count < old->shared_max)
return 0;
- } else
+ else
max = old->shared_max * 2;
} else
max = 4;
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