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

David Laight
2014-12-05T12:08:19Z

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