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

[Thomas Hellstrom <thellstrom@vmware.com>]

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@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?

/Thomas