Re: armv7-on-aarch64 stuck at urdlck

> On Jul 24, 2024, at 06:50, Konstantin Belousov <kib@freebsd.org> wrote:
> 
> On Wed, Jul 24, 2024 at 12:34:57PM +0200, mmel@freebsd.org wrote:
>> 
>> 
>> On 24.07.2024 12:24, Konstantin Belousov wrote:
>>> On Tue, Jul 23, 2024 at 08:11:13PM +0000, John F Carr wrote:
>>>> 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.
>>> 
>>> Lets not allow this to be lost.  Could anybody confirm that the patch
>>> below fixes the issue?
>>> 
>>> commit d560f4f6690a48476565278fd07ca131bf4eeb3c
>>> Author: Konstantin Belousov <kib@FreeBSD.org>
>>> Date:   Wed Jul 24 13:17:55 2024 +0300
>>> 
>>>     rtld: avoid division in __thr_map_stacks_exec()
>>>     The function is called by rtld with the rtld bind lock write-locked,
>>>     when fixing the stack permission during dso load.  Not every ARMv7 CPU
>>>     supports the div, which causes the recursive entry into rtld to resolve
>>>     the  __aeabi_uidiv symbol, causing self-lock.
>>>     Workaround the problem by using roundup2() instead of open-coding less
>>>     efficient formula.
>>>     Diagnosed by:   mmel
>>>     Based on submission by: John F Carr <jfc@mit.edu>
>>>     Sponsored by:   The FreeBSD Foundation
>>>     MFC after:      1 week
>>> 
> Just realized that it is wrong.  Stack size is user-controlled and it does
> not need to be power of two.

Your change is correct.  _thr_page_size is set to getpagesize(),
which is a power of 2.   The call to roundup2 takes a user-provided
size and rounds it up to a multiple of the system page size.

I tested the change and it works.  My change also works and
should compile to identical code.  I forgot there was a standard
function to do the rounding.

> For final resolving of deadlocks, after a full day of digging, I'm very much
>> incline  of adding -znow to the linker flags for libthr.so (and maybe also
>> for ld-elf.so). The runtime cost of resolving all symbols at startup is very
>> low. Direct pre-solving in _thr_rtld_init() is problematic for the _aeabi_*
>> symbols, since they don't have an official C prototypes, and some are not
>> compatible with C calling conventions.
> I do not like it. `-z now' changes (breaks) the ABI and makes some symbols
> not preemtible.
> 
> In the worst case, we would need a call to the asm routine which causes the
> resolution of the _eabi_* symbols on arm.
> 

It would also be possible to link libthr with libgcc.a and use a linker map
to hide the _eabi_ symbols.