Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available

[Christian KÃnig]

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;
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel