drm/amdgpu: fix fdinfo race with process exit

Get process vm root BO ref in case process is exiting and root BO is
freed, to avoid NULL pointer dereference backtrace:

BUG: unable to handle kernel NULL pointer dereference at
0000000000000000
Call Trace:
amdgpu_show_fdinfo+0xfe/0x2a0 [amdgpu]
seq_show+0x12c/0x180
seq_read+0x153/0x410
vfs_read+0x91/0x140[ 3427.206183]  ksys_read+0x4f/0xb0
do_syscall_64+0x5b/0x1a0
entry_SYSCALL_64_after_hwframe+0x65/0xca

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c

@@ -59,6 +59,7 @@ void amdgpu_show_fdinfo(struct seq_file *m, struct file *f)
 	uint64_t vram_mem = 0, gtt_mem = 0, cpu_mem = 0;
 	struct drm_file *file = f->private_data;
 	struct amdgpu_device *adev = drm_to_adev(file->minor->dev);
+	struct amdgpu_bo *root;
 	int ret;
 
 	ret = amdgpu_file_to_fpriv(f, &fpriv);
@@ -69,13 +70,19 @@ void amdgpu_show_fdinfo(struct seq_file *m, struct file *f)
 	dev = PCI_SLOT(adev->pdev->devfn);
 	fn = PCI_FUNC(adev->pdev->devfn);
 
-	ret = amdgpu_bo_reserve(fpriv->vm.root.bo, false);
+	root = amdgpu_bo_ref(fpriv->vm.root.bo);
+	if (!root)
+		return;
+
+	ret = amdgpu_bo_reserve(root, false);
 	if (ret) {
 		DRM_ERROR("Fail to reserve bo\n");
 		return;
 	}
 	amdgpu_vm_get_memory(&fpriv->vm, &vram_mem, &gtt_mem, &cpu_mem);
-	amdgpu_bo_unreserve(fpriv->vm.root.bo);
+	amdgpu_bo_unreserve(root);
+	amdgpu_bo_unref(&root);
+
 	seq_printf(m, "pdev:\t%04x:%02x:%02x.%d\npasid:\t%u\n", domain, bus,
 			dev, fn, fpriv->vm.pasid);
 	seq_printf(m, "vram mem:\t%llu kB\n", vram_mem/1024UL);