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Am 05.05.20 um 16:58 schrieb Felix
Kuehling:
Am 2020-05-05 um 3:47 a.m. schrieb
Christian König:
Just to reply here once more, this
patch is a clear NAK.
Agreed. But see below. I don't think all is well.
The references are grabbed in the call path of
drm_gem_prime_export() and dropped again in
drm_gem_dmabuf_release().
So they are perfectly balanced as far as I can see.
That is true for the GEM object references. But I believe there
is still a problem with the TTM BO references.
As far as I can tell amdgpu_bo_unref can free the TTM BO while
there are still references to the GEM object from DMA buf
exports. I think that's a fundamental problem with how we have
two reference counts for the same physical object (TTM BO and
the embedded GEM BO).
Completely agree, I also mentioned that problem during my talk on
FOSDEM. But calling amdgpu_bo_unref() to often is a bug in itself.
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.
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.
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@xxxxxxxxx>
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@xxxxxxxxx>
---
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;
}
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