[ Trimmed the cc list because my SMTP didn't accept that many addresses. ] On Sun, 26 Aug 2018 13:25:14 -0700 Linus Torvalds <torvalds at linux-foundation.org> wrote: > On Sun, Aug 26, 2018 at 12:32 PM H. Peter Anvin <hpa at zytor.com> wrote: > > > > Here is a full-blown (user space) test program demonstrating the whole > > technique and how to use it. > > So while I agree that some _THIS_IP_ users might be better off being > converted to __builtin_return_address(0) at the caller, I also think > that the whole "notailcall" thing shows why that can easily be more > problematic than just our currnet _THIS_IP_ solution. > > 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. > > - 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. > > 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? Seems nice. Do you even need this unique ID thing? AFAIKS the name would never really be useful. It could perhaps go into a cold data section too, I assume the common case is that you do not access it. Although gcc will end up putting the file and function names into regular rodata. Possibly we could add a printk specifier for it, pass it through to existing BUG, etc macros that want exactly this, etc. Makes a lot of sense. Thanks, Nick