[PATCH 1/1] drm/amdgpu: Take a reference to an exported BO
Christian König
christian.koenig at amd.com
Wed May 6 07:15:40 UTC 2020
Am 05.05.20 um 19:29 schrieb Felix Kuehling:
>>
>> What we could probably do to detect this is adding a BUG_ON() in TTMs
>> release function and checking if the GEM reference count is really dead.
>
> The problem is, that we have to guess whether there are still any
> dmabuf references to the GEM BO. There is no way amdgpu can know that.
> You can't make amdgpu responsible for keeping a reference to the TTM
> BO while the GEM BO is still referenced by entities completely out of
> the control of amdgpu.
>
That sounds like you don't understand how this works on the graphics
side, so here is a brief overview once more.
The last object still around is always the TTM BO, which can even be
resurrected from the dead (kref_init() called again) if necessary for
delayed delete.
Then we have the GEM object which holds a reference to the TTM BO. This
reference gets dropped when the GEM object gets destroyed.
Then we optionally have the DMA-buf object for exported BOs which holds
a reference to the GEM object and the driver.
This construct guarantees that the GEM object is never destroyed nor the
driver unloaded before the DMA-buf object referencing it is destroyed.
Additional to that the reference from the GEM object to the TTM BO
guarantees that the TTM BO is never destroyed before the GEM object is
destroyed.
> Another weird thing I see is that amdgpu_gem_free_object calls
> amdgpu_bo_unref. That implies that the GEM object conceptually holds a
> reference to the amdpu/TTM BO. But that is not really the case. Amdgpu
> never takes that reference that GEM is supposed to own. If it did, we
> would leak all our memory because nobody would ever drop that reference.
See amdgpu_bo_do_create(), the GEM object is initialized with
drm_gem_private_object_init() with a reference count of 1. Then the TTM
BO is initialized with ttm_bo_init_reserved() with a reference count of 1.
In general there are two use cases here, the first one is userspace
allocations with a GEM object handle. In this case the initial reference
is owned by the GEM object and dropped in amdgpu_gem_free_object().
The other use case are kernel internal allocations like page tables and
other general buffers. In this case the GEM object is ignored and the
last TTM BO reference is dropped by calling amdgpu_bo_unref().
>>> I think the correct solution is for amdgpu_bo_ref/unref to delegate
>>> its reference counting to drm_gem_object_get/put instead of
>>> ttm_bo_get/put. The amdgpu BO would hold one token reference to the
>>> TTM BO, which it can drop when the GEM BO refcount drops to 0.
>>> Finally, the amdgpu BO should only be freed once the TTM BO refcount
>>> also becomes 0.
>>
>> Just the other way around, but yes the long term plan should probably
>> be to merge the two.
>
> I need a short term solution. Because I have a bug that causes a
> kernel oops with applications that are valid and correct, as far as I
> can tell.
>
> I'm thinking a solution that doesn't require major changes to the way
> TTM and GEM interact would put amdgpu in charge of coordinating the
> two. Unfortunately that would mean adding a third reference count in
> amdgpu_bo, in addition to the ones in TTM and GEM. The amdgpu BO would
> hold one token reference to each of the GEM and TTM BO. When amdgpu
> refcount goes to 0 it releases that GEM BO token reference. When the
> GEM BO refcount goes to 0, we get a callback amdgpu_gem_object_free.
> There we can drop the token reference to the TTM BO. Once the TTM BO
> reference goes to 0 we free the memory.
>
> Does this sound feasible?
>
Well of hand it sounds like it makes the whole thing much more
complicated without any gain. I probably still haven't understood what
the core problem here is.
Regards,
Christian.
> Regards,
> Felix
>
>
>>
>> The difficult is currently we have a mismatch what locks could be
>> taken when we drop the references.
>>
>> Regards,
>> Christian.
>>
>>> Regards,
>>> Felix
>>>
>>>
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>> Am 01.05.20 um 16:44 schrieb Felix Kuehling:
>>>>>
>>>>> [dropping my gmail address]
>>>>>
>>>>> We saw this backtrace showing the call chain while investigating a
>>>>> kernel oops caused by this issue on the DKMS branch with the KFD
>>>>> IPC API. It happens after a dma-buf file is released with fput:
>>>>>
>>>>> [ 1255.049330] BUG: kernel NULL pointer dereference, address: 000000000000051e
>>>>> [ 1255.049727] #PF: supervisor read access in kernel mode
>>>>> [ 1255.050092] #PF: error_code(0x0000) - not-present page
>>>>> [ 1255.050416] PGD 0 P4D 0
>>>>> [ 1255.050736] Oops: 0000 [#1] SMP PTI
>>>>> [ 1255.051060] CPU: 27 PID: 2292 Comm: kworker/27:2 Tainted: G OE 5.3.0-46-generic #38~18.04.1-Ubuntu
>>>>> [ 1255.051400] Hardware name: Supermicro SYS-4029GP-TRT2/X11DPG-OT-CPU, BIOS 3.0a 02/26/2019
>>>>> [ 1255.051752] Workqueue: events delayed_fput
>>>>> [ 1255.052111] RIP: 0010:drm_gem_object_put_unlocked+0x1c/0x70 [drm]
>>>>> [ 1255.052465] Code: 4d 80 c8 ee 0f 0b eb d8 66 0f 1f 44 00 00 0f 1f 44 00 00 48 85 ff 74 34 55 48 89 e5 41 54 53 48 89 fb 48 8b 7f 08 48 8b 47 20 <48> 83 b8 a0 00 00 00 00 74 1a 4c 8d 67 68 48 89 df 4c 89 e6 e8 9b
>>>>> [ 1255.053224] RSP: 0018:ffffb4b62035fdc8 EFLAGS: 00010286
>>>>> [ 1255.053613] RAX: 000000000000047e RBX: ffff9f2add197850 RCX: 0000000000000000
>>>>> [ 1255.054032] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9f2aa2548aa0
>>>>> [ 1255.054440] RBP: ffffb4b62035fdd8 R08: 0000000000000000 R09: 0000000000000000
>>>>> [ 1255.054860] R10: 0000000000000010 R11: ffff9f2a4b1cc310 R12: 0000000000080005
>>>>> [ 1255.055268] R13: ffff9f2a4b1cc310 R14: ffff9f4e369161e0 R15: ffff9f2a1b2f9080
>>>>> [ 1255.055674] FS: 0000000000000000(0000) GS:ffff9f4e3f740000(0000) knlGS:0000000000000000
>>>>> [ 1255.056087] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> [ 1255.056501] CR2: 000000000000051e CR3: 00000002df00a004 CR4: 00000000007606e0
>>>>> [ 1255.056923] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>> [ 1255.057345] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>> [ 1255.057763] PKRU: 55555554
>>>>> [ 1255.058179] Call Trace:
>>>>> [ 1255.058603] drm_gem_dmabuf_release+0x1a/0x30 [drm]
>>>>> [ 1255.059025] dma_buf_release+0x56/0x130
>>>>> [ 1255.059443] __fput+0xc6/0x260
>>>>> [ 1255.059856] delayed_fput+0x20/0x30
>>>>> [ 1255.060272] process_one_work+0x1fd/0x3f0
>>>>> [ 1255.060686] worker_thread+0x34/0x410
>>>>> [ 1255.061099] kthread+0x121/0x140
>>>>> [ 1255.061510] ? process_one_work+0x3f0/0x3f0
>>>>> [ 1255.061923] ? kthread_park+0xb0/0xb0
>>>>> [ 1255.062336] ret_from_fork+0x35/0x40
>>>>>
>>>>> drm_gem_object_put_unlocked calls drm_gem_object_free when the
>>>>> obj->refcount reaches 0. From there it calls
>>>>> dev->driver->gem_free_object_unlocked, which is
>>>>> amdgpu_gem_object_free in amdgpu.
>>>>>
>>>>> Regards,
>>>>> Felix
>>>>>
>>>>> Am 2020-05-01 um 10:29 a.m. schrieb Christian König:
>>>>>> Am 01.05.20 um 16:21 schrieb Felix Kuehling:
>>>>>>> From: Felix Kuehling <felix.kuehling at gmail.com>
>>>>>>>
>>>>>>> That reference gets dropped when the the dma-buf is freed. Not
>>>>>>> incrementing
>>>>>>> the refcount can lead to use-after-free errors.
>>>>>>>
>>>>>>> Signed-off-by: Felix Kuehling <felix.kuehling at gmail.com>
>>>>>>> ---
>>>>>>> drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 9 ++++++++-
>>>>>>> 1 file changed, 8 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> index ffeb20f11c07..a0f9b3ef4aad 100644
>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>>>>>> @@ -398,8 +398,15 @@ struct dma_buf
>>>>>>> *amdgpu_gem_prime_export(struct drm_gem_object *gobj,
>>>>>>> return ERR_PTR(-EPERM);
>>>>>>> buf = drm_gem_prime_export(gobj, flags);
>>>>>>> - if (!IS_ERR(buf))
>>>>>>> + if (!IS_ERR(buf)) {
>>>>>>> buf->ops = &amdgpu_dmabuf_ops;
>>>>>>> + /* GEM needs a reference to the underlying object
>>>>>>> + * that gets dropped when the dma-buf is released,
>>>>>>> + * through the amdgpu_gem_object_free callback
>>>>>>> + * from drm_gem_object_put_unlocked.
>>>>>>> + */
>>>>>>> + amdgpu_bo_ref(bo);
>>>>>>> + }
>>>>>>
>>>>>> Of hand that doesn't sounds correct to me. Why should the
>>>>>> exported bo be closed through amdgpu_gem_object_free()?
>>>>>>
>>>>>> Regards,
>>>>>> Christian.
>>>>>>
>>>>>>> return buf;
>>>>>>> }
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> amd-gfx mailing list
>>>>> amd-gfx at lists.freedesktop.org
>>>>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>>>>
>>
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