Re: armv7-on-aarch64 stuck at urdlck

On Tue, Jul 23, 2024 at 07:46:51PM +0200, Michal Meloun wrote:
> 
> 
> On 23.07.2024 11:36, Konstantin Belousov wrote:
> > On Tue, Jul 23, 2024 at 09:53:41AM +0200, Michal Meloun wrote:
> > > The good news is that I'm finally able to generate a working/locking
> > > test case.  The culprit (at least for me) is if "-mcpu" is used when
> > > compiling libthr (e.g. indirectly injected via CPUTYPE in /etc/make.conf).
> > > If it is not used, libthr is broken (regardless of -O level or debug/normal
> > > build), but -mcpu=cortex-a15 will always produce a working libthr.
> > 
> > I think this is very significant progress.
> > 
> > Do you plan to drill down more to see what is going on?
> 
> So the problem is now clear, and I fear it may apply to other architectures
> as well.
> dlopen_object() (from rtld_elf),
> https://cgit.freebsd.org/src/tree/libexec/rtld-elf/rtld.c#n3766,
> holds the rtld_bind_lock write lock for almost the entire time a new library
> is loaded.
> If the code uses a yet unresolved symbol to load the library, the rtl_bind()
> function attempts to get read lock of  rtld_bind_lock and a deadlock occurs.
> 
> In this case, it round_up() in _thr_stack_fix_protection,
> https://cgit.freebsd.org/src/tree/lib/libthr/thread/thr_stack.c#n136.
> Issued by __aeabi_uidiv (since not all armv7 processors support HW divide).
> 
> Unfortunately, I'm not sure how to fix it.  The compiler can emit __aeabi_<>
> in any place, and I'm not sure if it can resolve all the symbols used by
> rtld_eld and libthr beforehand.

This is a common issue, we pre-resolve a lot of symbols to avoid lock
recursion.  Please take a look at lib/libthr/thread/thr_rtld.c
_thr_rtld_init().