[PATCH 2/5] drm: Break out ioctl permission check to a separate function

[David Herrmann]

Hi

On Thu, Mar 13, 2014 at 1:11 PM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> Hi.
>
> Thanks for reviewing. I'll incorporate your suggestions, except this
> one, and resend.
>
>
> On 03/13/2014 12:19 PM, David Herrmann wrote:
>> Hi
>>
>> On Thu, Mar 13, 2014 at 11:57 AM, Thomas Hellstrom
>> <thellstrom at vmware.com> wrote:
> ...
>
> -               if (cmd & IOC_IN) {
> -                       if (copy_from_user(kdata, (void __user *)arg,
> -                                          usize) != 0) {
> -                               retcode = -EFAULT;
> +       retcode = drm_ioctl_permit(ioctl->flags, file_priv);
> +       if (unlikely(retcode))
>
>> That "unlikely" seems redundant given that all error paths in
>> drm_ioctl_permit() already are "unlikely".
>
> Yes, we know that's true, but I don't think compilers in general can
> combine branch prediction hints in that way,
> or even have the information necessary to do it.
> I mean even if each individual test resulting in an error is unlikely,
> how could the compiler know that
> all tests combined would result in an error being unlikely?

The function is static, so the compiler can see that it returns "!=0"
only if one of the "unlikely" branches was hit. So I think it's safe
to assume the whole thing returns "!=0" only in unlikely conditions.
But it's probably inlined, anyway..

I'm no big fan of excessive likely/unlikely annotations, but I'm fine
if you want to keep it.

Thanks
David