Re: armv7-on-aarch64 stuck at urdlck

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.

> 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.

> 
> Warner, Konstantin, could you please comment on this?
> 
> 
> Michal