Date: Wed, 24 Jul 2024 19:34:47 +0200 From: "mmel@freebsd.org" <meloun.michal@gmail.com> To: Konstantin Belousov <kib@freebsd.org>, John F Carr <jfc@mit.edu> Cc: Mark Millard <marklmi@yahoo.com>, FreeBSD Current <freebsd-current@freebsd.org>, "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: armv7-on-aarch64 stuck at urdlck Message-ID: <28484869-05fd-4391-9501-10b93280f7a4@freebsd.org> In-Reply-To: <ZqEiBJF7UxcUWCQ2@kib.kiev.ua> References: <33251aa3-681f-4d17-afe9-953490afeaf0@gmail.com> <0DD19771-3AAB-469E-981B-1203F1C28233@yahoo.com> <be023545-2b25-49ec-b6f1-9e05cd402646@gmail.com> <Zp95qtxK0CeDdp-d@kib.kiev.ua> <6a969609-fa0e-419d-83d5-e4fcf0f6ec35@freebsd.org> <FABF7440-70D2-4BAB-8B0B-4CA950CFFA60@mit.edu> <ZqDWSU_h5J1fYCrz@kib.kiev.ua> <f39b16b5-bbfb-4011-92fb-834330841533@freebsd.org> <ZqDcamh6r3B-oEB-@kib.kiev.ua> <A7348370-0BEE-4EA4-8521-03C07F025F40@mit.edu> <ZqEiBJF7UxcUWCQ2@kib.kiev.ua>
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On 24.07.2024 17:47, Konstantin Belousov wrote: > On Wed, Jul 24, 2024 at 01:07:39PM +0000, John F Carr wrote: >> >> >>> 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. > Right, my bad, thank you for correcting my thinko. > >> >>> 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. > In principle yes, but if the ARM ABI states that _eabi symbols must be used, > and exported from libc, then this is also some form of ABI breakage. I hope that https://reviews.freebsd.org/D46104 is acceptable :)
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