Re: armv7-on-aarch64 stuck at urdlck

On Jul 23, 2024, at 13:46, Michal Meloun <meloun.michal@gmail.com> 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.
> 
> 
> Michal
> 

In this case (but not for all _aeabi_ functions) we can avoid division
as long as page size is a power of 2.

The function is

  static inline size_t
  round_up(size_t size)
  {
  	if (size % _thr_page_size != 0)
  		size = ((size / _thr_page_size) + 1) *
  		    _thr_page_size;
  	return size;
  }

The body can be condensed to

  return (size + _thr_page_size - 1) & ~(_thr_page_size - 1);

This is shorter in both lines of code and instruction bytes.

John Carr