Messages in this thread | | | Subject | Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available | From | Christian König <> | Date | Mon, 5 Mar 2018 12:51:07 +0100 |
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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: 2018年3月5日 19:39 > To: Liu, Monk <Monk.Liu@amd.com>; dri-devel@lists.freedesktop.org; linux-kernel@vger.kernel.org > 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: 2018年3月5日 19:29 >> To: Liu, Monk <Monk.Liu@amd.com>; dri-devel@lists.freedesktop.org; >> linux-kernel@vger.kernel.org >> 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 König [mailto:ckoenig.leichtzumerken@gmail.com] >>> Sent: 2018年3月5日 19:22 >>> To: Liu, Monk <Monk.Liu@amd.com>; Koenig, Christian >>> <Christian.Koenig@amd.com>; dri-devel@lists.freedesktop.org; >>> linux-kernel@vger.kernel.org >>> 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 König [mailto:ckoenig.leichtzumerken@gmail.com] >>>> Sent: 2018年2月28日 16:27 >>>> To: Liu, Monk <Monk.Liu@amd.com>; dri-devel@lists.freedesktop.org; >>>> linux-kernel@vger.kernel.org >>>> 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: >>>>> 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@amd.com> >>>>> --- >>>>> 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@lists.freedesktop.org >>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel > _______________________________________________ > dri-devel mailing list > dri-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel
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