On Jul 23, 2024, at 13:46, Michal Meloun <meloun.mic...@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
