[PATCH] drm/virtio: fix virtio-gpu resource id creation race

[Gurchetan Singh]

On Wed, Feb 19, 2020 at 4:20 PM John Bates <jbates at google.com> wrote:
>
>
>
> On Wed, Feb 19, 2020 at 12:40 PM John Bates <jbates at chromium.org> wrote:
>>
>> The previous code was not thread safe and caused
>> undefined behavior from spurious duplicate resource IDs.
>> In this patch, an atomic_t is used instead. We no longer
>> see any duplicate IDs in tests with this change.
>>
>> Signed-off-by: John Bates <jbates at chromium.org>
>> ---
>>
>>  drivers/gpu/drm/virtio/virtgpu_object.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
>> index 017a9e0fc3bb..b11c1fce1770 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_object.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_object.c
>> @@ -42,8 +42,8 @@ static int virtio_gpu_resource_id_get(struct virtio_gpu_device *vgdev,
>>                  * "f91a9dd35715 Fix unlinking resources from hash
>>                  * table." (Feb 2019) fixes the bug.
>>                  */
>> -               static int handle;
>> -               handle++;
>> +               static atomic_t seqno;

nit: ATOMIC_INIT(0)?

>> +               int handle = atomic_inc_return(&seqno);
>>                 *resid = handle + 1;
>>         } else {
>>                 int handle = ida_alloc(&vgdev->resource_ida, GFP_KERNEL);
>> --
>> 2.25.0.265.gbab2e86ba0-goog
>>