Re: [PATCH v4] eliminate mutex in fast path of __register_frame

[Stephan Bergmann <sbergman@redhat.com>]

On 16/09/2022 12:19, Thomas Neumann via Gcc-patches wrote:
> The __register_frame/__deregister_frame functions are used to register
> unwinding frames from JITed code in a sorted list. That list itself
> is protected by object_mutex, which leads to terrible performance
> in multi-threaded code and is somewhat expensive even if single-threaded.
> There was already a fast-path that avoided taking the mutex if no
> frame was registered at all.
> 
> This commit eliminates both the mutex and the sorted list from
> the atomic fast path, and replaces it with a btree that uses
> optimistic lock coupling during lookup. This allows for fully parallel
> unwinding and is essential to scale exception handling to large
> core counts.

I haven't debugged this in any way, nor checked whether it only impacts 
exactly my below scenario, but noticed the following:

At least when building LibreOffice with Clang (16 trunk) with ASan and 
UBsan enabled against libstdc++ (with --gcc-toolchain and 
LD_LIBRARY_PATH to pick up a libstdc++ trunk build including this change 
at build and run-time), at least one of the LibreOffice tests executed 
during the build started to fail with

> Thread 1 "cppunittester" received signal SIGABRT, Aborted.
> __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0)Downloading 0.00 MB source file /usr/src/debug/glibc-2.36-4.fc37.x86_64/nptl/pthread_kill.c
>  at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##nptl##pthread_kill.c:44
> 44	      return INTERNAL_SYSCALL_ERROR_P (ret) ? INTERNAL_SYSCALL_ERRNO (ret) : 0;
> (gdb) bt
> #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##nptl##pthread_kill.c:44
> #1  0x00007ffff6dcdd33 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##nptl##pthread_kill.c:78
> #2  0x00007ffff6d7daa6 in __GI_raise (sig=sig@entry=6) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##signal##..##sysdeps##posix##raise.c:26
> #3  0x00007ffff6d677fc in __GI_abort () at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##stdlib##abort.c:79
> #4  0x00007ffff6f377e8 in __deregister_frame_info_bases (begin=<optimized out>) at ~/gcc/trunk/src/libgcc/unwind-dw2-fde.c:285
> #5  __deregister_frame_info_bases (begin=<optimized out>) at ~/gcc/trunk/src/libgcc/unwind-dw2-fde.c:223
> #6  0x00007fffc7c3b53f in __do_fini () at ~/lo/core/instdir/program/libcairo.so.2
> #7  0x00007ffff7fcda9e in _dl_fini () at ~/.cache/debuginfod_client/653dfb54d6e6d9c27c349f698a8af1ab86d5501d/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##elf##dl-fini.c:142
> #8  0x00007ffff6d7ff35 in __run_exit_handlers (status=0, listp=0x7ffff6f13840 <__exit_funcs>, run_list_atexit=run_list_atexit@entry=true, run_dtors=run_dtors@entry=true) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##stdlib##exit.c:113
> #9  0x00007ffff6d800b0 in __GI_exit (status=<optimized out>) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##stdlib##exit.c:143
> #10 0x00007ffff6d68517 in __libc_start_call_main (main=main@entry=0x5555556c9ef0 <main(int, char**)>, argc=argc@entry=24, argv=argv@entry=0x7ffffffefbf8) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##csu##..##sysdeps##nptl##libc_start_call_main.h:74
> #11 0x00007ffff6d685c9 in __libc_start_main_impl (main=0x5555556c9ef0 <main(int, char**)>, argc=24, argv=0x7ffffffefbf8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffffffefbe8) at ~/.cache/debuginfod_client/a6572cd46182057d3dbacf1685a12edab0e2eda1/source##usr##src##debug##glibc-2.36-4.fc37.x86_64##csu##..##csu##libc-start.c:381
> #12 0x00005555555f1575 in _start ()

and which went away again when locally reverting this 
<https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=6e80a1d164d1f996ad08a512c000025a7c2ca893> 
"eliminate mutex in fast path of __register_frame".