On Sun, Aug 26, 2018 at 1:25 PM Linus Torvalds
[off-list ref] wrote:

Honestly, I'd suggest:

 - just do the current_text_addr() to _THIS_IP_ conversion

 - keep _THIS_IP_ and make it be the generic one, and screw the whole
"some architectures might implement is better" issue. Nobody cares.

And mention it to the compiler vendors as this seems like a case where
code gen can be improved.

 - try to convince people to move away from the "we want the kernel
instruction pointer for the call" model entirely, and consider this a
"legacy" issue.

The whole instruction pointer is a nasty thing. We should discourage
it and not make complex infrastructure for it.

Yes, please.  I think we should strive for simplicity here.

Instead, maybe we could encourage something like

  struct kernel_loc { const char *file; const char *fn; int line; };

  #define __GEN_LOC__(n) \
        ({ static const struct kernel_loc n = { \
                __FILE__, __FUNCTION__, __LINE__  \
           }; &n; })

  #define _THIS_LOC_ __GEN_LOC__(__UNIQUE_ID(loc))

which is a hell of a lot nicer to use, and actually allows gcc to
optimize things (try it: if you pass a _THIS_LOC_ off to an inline
function, and that inline function uses the name and line number, gcc
will pick them up directly, without the extra structure dereference.

Wouldn't it be much nicer to pass these kinds of "location pointer"
around, rather than the nasty _THIS_IP_ thing?

Certainly lockdep looks like it could easily take that "const struct
kernel_loc *" instead of "unsigned long ip". Makes it easy to print
out the lockdep info.

Ok, I didn't try to convert anybody, so maybe people who currently use
_THIS_IP_ or current_text_addr() have some fundamental reason why they
want just that, but let's not male _THIS_IP_ more complex than it
needs to be.

Hmm?

             Linus

This is extremely reasonable.  I can follow up with the lockdep folks
to see if they really need _THIS_IP_ to solve their problem, or if
there's a simpler solution that can solve their needs.  Sometimes
taking a step back and asking for clarity around the big picture
allows simpler solutions to shake out.
-- 
Thanks,
~Nick Desaulniers