Re: Reviving planned ldso changes

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2017-01-03 00:43:51 -0500]:
> One item that's been on the agenda for a long time (maybe 1.5 years
> now?) is planned dynamic linker improvements. The thread "Further
> dynamic linker optimizations" from summer 2015 covered parts but maybe
> not all of what I had in mind.
> 
> - Instead of a global flag for each library, have a separate linked
>   list for globally visible libraries. This will speed up symbol
>   lookup (by skipping non-global ones, and skipping checking the
>   global flag) and also makes it so that load order searching can be
>   done based on the order in which the libraries became global, rather
>   than their original load order when they might have been non-global.
> 
> - Dependency-order ctor execution. I think the walk order looks
>   something like, starting at a given node (initially the main app or
>   new library being loaded dynamically), traversing to its first
>   deps[] entry that hasn't been constructed, or, if none remain,
>   executing its ctors then traversing back to its needed_by. This
>   process avoids the need for any call recursion and should be
>   near-optimal (if not optimal) provided the position in a dso's
>   deps[] list is saved in the dso struct.
> 
> - Recursive dlopen improvements. The thread "dlopen deadlock" from
>   January 2016 covers some of the issues. I don't think we reached a
>   complete conclusion/solution, but for reasonable cases where it's
>   clear that dlopen *should* work without deadlock, I think we have a
>   proposed design that works.
> 
> - Possibly other optimizations I've left out...

- symbol lookup logic is duplicated between do_dlsym and find_sym,
  it would be nice to do that with common code path if possible.