Re: [PATCH v1 3/5] mm, uaccess: trigger might_sleep() in might_fault() when pagefaults are disabled

David Hildenbrand
2014-12-05T13:30:23Z

> From: David Hildenbrand [...
> > > This should be likely() instead of unlikely(), no?
> > > I'd rather write this
> > >
> > > if (pagefault_disabled())
> > > return;
> > > __might_sleep(file, line, 0);
> > >
> > > and leave the likely stuff completely away.
> >
> > Makes perfect sense!
>
> From my experience of getting (an older version of) gcc to emit
> 'correctly' statically predicted branches I found that code that
> looks like (I don't think return/goto make any difference):
>
> If (unlikely(condition)) {
> code;
> }
> more_code;
>
> is compile with a forwards conditional branch (ie ignoring the unlikely()).
> Similarly 'if () continue' is likely to generate a 'predicted taken'
> backwards conditional branch.
>
> To get the desired effect you need a non-empty 'else' part, an assembler
> comment will suffice, eg: asm volatile("# comment").
>
> David
>
>
>

Thanks for the hint David!

I'm going to drop that unlikely and simply replace in_atomic() by
pagefault_disabled() - will also keep the change minimal!

David