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Crash when using _Py_DumpTracebackThreads #128400
Description
Crash report
What happened?
# triggering program
# based on test.test_faulthandler.FaultHandlerTests.test_dump_traceback_threads
import faulthandler
from threading import Thread, Event
class Waiter(Thread):
def __init__(self):
Thread.__init__(self)
self.running = Event()
self.stop = Event()
def run(self):
self.running.set()
self.stop.wait()
def main():
for i in range(100):
waiter = Waiter()
waiter.start()
waiter.running.wait()
faulthandler.dump_traceback(all_threads=True)
waiter.stop.set()
waiter.join()
if __name__ == '__main__':
main()This will case the interpreter to segfault if built with --disable-gil --with-pydebug. I've bisected it to this commit:
Using gdb:
Thread 1 "python" received signal SIGSEGV, Segmentation fault.
_PyFrame_GetCode (f=f@entry=0x7ffff7fb32d8) at ./Include/internal/pycore_frame.h:83
83 assert(PyCode_Check(executable));
(gdb) p executable
$7 = 0x0
(gdb) bt
#0 _PyFrame_GetCode (f=f@entry=0x7ffff7fb32d8) at ./Include/internal/pycore_frame.h:83
#1 PyUnstable_InterpreterFrame_GetLine (frame=frame@entry=0x7ffff7fb32d8) at Python/frame.c:149
#2 0x000055555590e9eb in dump_frame (fd=2, frame=0x7ffff7fb32d8) at Python/traceback.c:905
#3 dump_traceback (fd=fd@entry=2, tstate=tstate@entry=0x555555cef140, write_header=write_header@entry=0) at Python/traceback.c:974
#4 0x000055555590ed1c in _Py_DumpTracebackThreads (fd=2, interp=<optimized out>, interp@entry=0x0, current_tstate=0x555555c5dea8 <_PyRuntime+359528>) at Python/traceback.c:1090
#5 0x0000555555924489 in faulthandler_dump_traceback_py (self=<optimized out>, args=<optimized out>, kwargs=<optimized out>) at ./Modules/faulthandler.c:240
#6 0x00005555556feae1 in cfunction_call (func=func@entry=<built-in method dump_traceback of module object at remote 0x20000778900>, args=args@entry=(), kwargs=kwargs@entry={'all_threads': True})
[...]
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
Output from running 'python -VV' on the command line:
Python 3.14.0a0 experimental free-threading build (bisect/bad:b2afe2aae48, Jan 1 2025, 17:09:16) [Clang 19.1.6 (++20241217105838+657e03f8625c-1exp120241217105944.74)]
Linked PRs
- gh-128400: Stop-the-world when manually calling
faulthandler#128422 - [3.13] gh-128400: Stop-the-world when manually calling
faulthandler(GH-128422) #128423 - gh-128400: Only show the current thread in
faulthandlerif the GIL is disabled #128425 - gh-128400: Only show the current thread in
Py_FatalErroron the free-threaded build #128758
Activity
ZeroIntensity commentedon Jan 2, 2025
It's probably because accessing fields on another thread state isn't very safe, especially on free-threading. Maybe we should just make a stop-the-world pause for manual invocation of
faulthandler?colesbury commentedon Jan 2, 2025
Yes, I think that would make sense.
pythongh-128400: Stop the world when calling manually
gh-128400: Stop-the-world when manually calling `faulthandler` (GH-12…
pythongh-128400: Stop-the-world when manually calling `faulthandler` (p…
colesbury commentedon Jan 2, 2025
@ZeroIntensity - if you're interested in a related follow-up thread safety issue, I think it'd be helpful to make
faulthandleronly dump the current thread's stack when called from a signal handler. Basically, I think we should ignoreall_threads=Truein the free threading build because accessing other thread's stacks while they're running is likely to crash. It's not exactly thread-safe in the default build either, but it mostly works whereas in the free threading build it frequently crashes.ZeroIntensity commentedon Jan 2, 2025
Sure, I can do that.
[3.13] gh-128400: Stop-the-world when manually calling `faulthandler` (…
dgrisby commentedon Jan 2, 2025
Even with the GIL, faulthandler is not safe to use in a multi-threaded process: #116008
gh-128400: Only show the current thread in `faulthandler` if the GIL …
colesbury commentedon Jan 3, 2025
@dgrisby, yes, but there's a meaningful difference between "mostly works" and "mostly crashes". We'd like to get to the point where faulthandler works about as reliably in the free threading build as it does in the GIL-enabled build, even if that still has bugs.
colesbury commentedon Jan 3, 2025
This is fixed now
dgrisby commentedon Jan 3, 2025
If that meets your use case, I suppose. The environment I work in, we have long-lived services that must not crash. For us, "mostly works" is equivalent to "too dangerous to use".
colesbury commentedon Jan 4, 2025
The intention of my comment was to explain why we are bothering with fixes to faulthandler given it's limitations. I'm not suggesting that you use faulthandler.
pythongh-128400: Only show the current thread in `faulthandler` if th…
pythongh-128400: Stop-the-world when manually calling `faulthandler` (p…
pythongh-128400: Only show the current thread in `faulthandler` if th…
gh-128400: Only show the current thread in `Py_FatalError` on the fre…