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

[Iago Abal]

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
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