Potential BUG: [PATCH 2/8] drm/amdgpu: add a generic fb accessing helper function(v3)

[Luben Tuikov]

Hi Iago,

Thank you for finding and reporting this potential double lock.

Yes indeed, I see it--it can indeed happen.

Now, since the primitives used--macros using "amdgpu_mm_(r|w)reg\(.*\)"--in
"amdgpu_device_vram_access()" do use their own register-access spinlocks,
it maybe wise to remove the spinlock take/release in "amdgpu_device_vram_access()".

We'll look into it and possibly submit another patch.

Thanks again.

Regards,
Luben

On 2019-11-16 11:21 a.m., Iago Abal wrote:
> Hi,
> 
> With the help of a static bug finder (EBA - https://github.com/IagoAbal/eba) I have found a potential double lock in Linux Next tag next-20191115, file drivers/gpu/drm/amd/amdgpu/amdgpu_device.c.
> 
> This bug seems to be introduced by commit
> e35e2b117f4 ("drm/amdgpu: add a generic fb accessing helper function(v3)").
> 
> The steps to reproduce it would be:
> 
> 1. Start in function `amdgpu_device_vram_access`.
> 2. Enter for-loop `for (last += pos; pos <= last; pos += 4)`.
> 3. First lock: `spin_lock_irqsave(&adev->mmio_idx_lock, flags)`.
> 4. Call to `WREG32_NO_KIQ(mmMM_INDEX, ((uint32_t)pos) | 0x80000000)`.
>    5. Note `#define WREG32_NO_KIQ(reg, v) amdgpu_mm_wreg(adev, (reg), (v), AMDGPU_REGS_NO_KIQ)`.
>    6. Continue in function `amdgpu_mm_wreg`.
>    7. Take else-branch in the third if-statement.
>    8. Double lock: `spin_lock_irqsave(&adev->mmio_idx_lock, flags)`.
> 
> I think the control flow could reach that second lock, but you may know better.
> 
> Hope it helps!
> 
> -- iago