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test_threading.test_recursion_limit() does crash (segfault) on Alpine Linux (musl) #148260

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test_threading.test_recursion_limit() does crash (segfault) on Alpine Linux (musl)#148260

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interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)testsTests in the Lib/test dirTests in the Lib/test dirtopic-free-threading

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vstinner

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on Apr 8, 2026

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Python built with ./configure --with-pydebug --disable-gil and gcc -Og on Alpine Linux (Python built with musl C library).

Example of crash on "AMD64 Alpine Linux NoGIL 3.x" buildbot: https://buildbot.python.org/#/builders/1819/builds/1699

Script:

import threading

def recurse():
    return recurse()

def outer():
    try:
        recurse()
    except RecursionError:
        pass

w = threading.Thread(target=outer)
w.start()
w.join()
print('end of main thread')

gdb traceback:

(gdb) where
#0  0x0000560e9bedb871 in _Py_Dealloc (op=0x20004156010) at Objects/object.c:3277
#1  0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x20004156010) at Objects/object.c:444
#2  0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x20004156010) at ./Include/refcount.h:359
#3  0x0000560e9c088af7 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#4  0x0000560e9c088e30 in tb_dealloc (op=0x20004156070) at Python/traceback.c:245
#5  0x0000560e9bedb8e0 in _Py_Dealloc (op=0x20004156070) at Objects/object.c:3300
#6  0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x20004156070) at Objects/object.c:444
#7  0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x20004156070) at ./Include/refcount.h:359
#8  0x0000560e9c088af7 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#9  0x0000560e9c088e30 in tb_dealloc (op=0x200041560d0) at Python/traceback.c:245
#10 0x0000560e9bedb8e0 in _Py_Dealloc (op=0x200041560d0) at Objects/object.c:3300
#11 0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x200041560d0) at Objects/object.c:444
#12 0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x200041560d0) at ./Include/refcount.h:359
#13 0x0000560e9c088af7 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#14 0x0000560e9c088e30 in tb_dealloc (op=0x20004156130) at Python/traceback.c:245
#15 0x0000560e9bedb8e0 in _Py_Dealloc (op=0x20004156130) at Objects/object.c:3300
#16 0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x20004156130) at Objects/object.c:444
#17 0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x20004156130) at ./Include/refcount.h:359
#18 0x0000560e9c088af7 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#19 0x0000560e9c088e30 in tb_dealloc (op=0x20004156190) at Python/traceback.c:245
#20 0x0000560e9bedb8e0 in _Py_Dealloc (op=0x20004156190) at Objects/object.c:3300
#21 0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x20004156190) at Objects/object.c:444
#22 0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x20004156190) at ./Include/refcount.h:359
#23 0x0000560e9c088af7 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#24 0x0000560e9c088e30 in tb_dealloc (op=0x200041561f0) at Python/traceback.c:245
#25 0x0000560e9bedb8e0 in _Py_Dealloc (op=0x200041561f0) at Objects/object.c:3300
#26 0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x200041561f0) at Objects/object.c:444
#27 0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x200041561f0) at ./Include/refcount.h:359
#28 0x0000560e9c088af7 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#29 0x0000560e9c088e30 in tb_dealloc (op=0x20004156250) at Python/traceback.c:245
#30 0x0000560e9bedb8e0 in _Py_Dealloc (op=0x20004156250) at Objects/object.c:3300
#31 0x0000560e9bedb9ef in _Py_MergeZeroLocalRefcount (op=op@entry=0x20004156250) at Objects/object.c:444
#32 0x0000560e9c088aaf in Py_DECREF (filename=filename@entry=0x560e9c12425b "./Include/refcount.h", 
(...)
#3751 0x0000560e9be774fb in PyObject_Call (callable=<optimized out>, args=<optimized out>, kwargs=<optimized out>)
    at Objects/call.c:373
#3752 0x0000560e9c10f6ef in thread_run (boot_raw=boot_raw@entry=0x20000030920) at ./Modules/_threadmodule.c:388
#3753 0x0000560e9c0874c0 in pythread_wrapper (arg=<optimized out>) at Python/thread_pthread.h:234
#3754 0x00007f484b327573 in start (p=<optimized out>) at src/thread/pthread_create.c:207
#3755 0x00007f484b328ec1 in __clone () at src/thread/x86_64/clone.s:22
Backtrace stopped: frame did not save the PC

(gdb) info threads
  Id   Target Id                  Frame 
  1    LWP 9804 "python"          __cp_end () at src/thread/x86_64/syscall_cp.s:29
* 2    LWP 9807 "Thread-1 (outer" 0x0000560e9bedb871 in _Py_Dealloc (op=0x20004156010) at Objects/object.c:3277

_Py_Dealloc() has the Trashcan mechanism which is supposed to avoid long chain of _Py_Dealloc() calls to reduce the stack memory consumption. But it seems like it doesn't work on Alpine Linux (Python built with musl C library).

gdb variables in the _Py_Dealloc() frame:

(gdb) p type->tp_name
$6 = 0x555ba66f5c7f "traceback"

(gdb) p gc_flag
$7 = 16384

In the parent _Py_Dealloc() frame:

(gdb) p type->tp_name
$9 = 0x555ba66f5c7f "traceback"
(gdb) p gc_flag
$10 = <optimized out>
(gdb) p margin
$11 = 115

In the first _Py_Dealloc() frame (outer):

(gdb) p margin
$12 = 119
(gdb) p type->tp_name
$13 = 0x555ba66e9dcd "RecursionError"
(gdb) p gc_flag
$14 = <optimized out>
(gdb) p type->tp_flags & (1UL << 14)  # gc_flag
$15 = 16384

On a regular build (without Free Threading), the script works as expected, it displays:

end of main thread

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vstinner
added
testsTests in the Lib/test dirTests in the Lib/test dir

interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

topic-free-threading
on Apr 8, 2026

vstinner commented on Apr 8, 2026

vstinner

on Apr 8, 2026

MemberAuthor

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Ah, I can also trigger the bug on a regular build (not Free Threaded) on Alpine Linux, Python built with: ./configure --with-pydebug and gcc -Og. I just have to increase the recursion limit to 2 000 frames:

import threading
import sys

def recurse():
    return recurse()

def outer():
    try:
        recurse()
    except RecursionError:
        pass

sys.setrecursionlimit(2_000)
w = threading.Thread(target=outer)
w.start()
w.join()
print('end of main thread')

I get a similar gdb traceback:

#0  0x0000560e899e7155 in _PyObject_GC_UNTRACK (filename=filename@entry=0x560e89af12ee "Python/gc.c", 
    lineno=lineno@entry=2382, op=op@entry=0x7f5d79f06170) at ./Include/internal/pycore_gc.h:279
#1  0x0000560e899e9442 in PyObject_GC_UnTrack (op_raw=op_raw@entry=0x7f5d79f06170) at Python/gc.c:2382
#2  0x0000560e89a49d11 in tb_dealloc (op=0x7f5d79f06170) at Python/traceback.c:244
#3  0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f06170) at Objects/object.c:3300
#4  0x0000560e89a49b28 in Py_DECREF (filename=filename@entry=0x560e89ade25b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x7f5d79f06170) at ./Include/refcount.h:410
#5  0x0000560e89a49b47 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#6  0x0000560e89a49d1a in tb_dealloc (op=0x7f5d79f061c0) at Python/traceback.c:245
#7  0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f061c0) at Objects/object.c:3300
#8  0x0000560e89a49b28 in Py_DECREF (filename=filename@entry=0x560e89ade25b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x7f5d79f061c0) at ./Include/refcount.h:410
#9  0x0000560e89a49b47 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#10 0x0000560e89a49d1a in tb_dealloc (op=0x7f5d79f06210) at Python/traceback.c:245
#11 0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f06210) at Objects/object.c:3300
#12 0x0000560e89a49b28 in Py_DECREF (filename=filename@entry=0x560e89ade25b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x7f5d79f06210) at ./Include/refcount.h:410
#13 0x0000560e89a49b47 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#14 0x0000560e89a49d1a in tb_dealloc (op=0x7f5d79f06260) at Python/traceback.c:245
#15 0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f06260) at Objects/object.c:3300
#16 0x0000560e89a49b28 in Py_DECREF (filename=filename@entry=0x560e89ade25b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x7f5d79f06260) at ./Include/refcount.h:410
#17 0x0000560e89a49b47 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#18 0x0000560e89a49d1a in tb_dealloc (op=0x7f5d79f062b0) at Python/traceback.c:245
#19 0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f062b0) at Objects/object.c:3300
#20 0x0000560e89a49b28 in Py_DECREF (filename=filename@entry=0x560e89ade25b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x7f5d79f062b0) at ./Include/refcount.h:410
#21 0x0000560e89a49b47 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#22 0x0000560e89a49d1a in tb_dealloc (op=0x7f5d79f06300) at Python/traceback.c:245
#23 0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f06300) at Objects/object.c:3300
#24 0x0000560e89a49b28 in Py_DECREF (filename=filename@entry=0x560e89ade25b "./Include/refcount.h", 
    lineno=lineno@entry=520, op=0x7f5d79f06300) at ./Include/refcount.h:410
#25 0x0000560e89a49b47 in Py_XDECREF (op=<optimized out>) at ./Include/refcount.h:520
#26 0x0000560e89a49d1a in tb_dealloc (op=0x7f5d79f06350) at Python/traceback.c:245
#27 0x0000560e898a60b6 in _Py_Dealloc (op=op@entry=0x7f5d79f06350) at Objects/object.c:3300
...
#4128 0x0000560e8984887b in PyObject_Call (callable=<optimized out>, args=<optimized out>, kwargs=<optimized out>)
    at Objects/call.c:373
#4129 0x0000560e89aca755 in thread_run (boot_raw=boot_raw@entry=0x7f5d7a1cdd50) at ./Modules/_threadmodule.c:388
#4130 0x0000560e89a485c6 in pythread_wrapper (arg=<optimized out>) at Python/thread_pthread.h:234
#4131 0x00007f5d7a3e9573 in start (p=<optimized out>) at src/thread/pthread_create.c:207
#4132 0x00007f5d7a3eaec1 in __clone () at src/thread/x86_64/clone.s:22
Backtrace stopped: frame did not save the PC

First _Py_Dealloc() call:

(gdb) p type->tp_name
$1 = 0x560e89ae4d03 "RecursionError"
(gdb) p type->tp_flags & (1UL << 14)  # gc_flag
$3 = 16384
(gdb) p margin
$4 = 119

Last _Py_Dealloc() call:

(gdb) p type->tp_name
$5 = 0x560e89af0a40 "traceback"
(gdb) p type->tp_flags & (1UL << 14)  # gc_flag
$7 = 16384
(gdb) p margin
$8 = 115

vstinner
changed the title ~~[-]test_threading.test_recursion_limit() does crash (segfault) on Alpine with Free Threaded build[/-]~~ [+]test_threading.test_recursion_limit() does crash (segfault) on Alpine Linux (musl)[/+] on Apr 8, 2026
vstinner commented on Apr 8, 2026
vstinner
on Apr 8, 2026
MemberAuthor
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https://wiki.musl-libc.org/functional-differences-from-glibc.html says:

musl provides a default thread stack size of 128k

On my Linux x86-64 (Fedora 43) with glibc, I get a default stack size of 8 MiB.

We can add -Wl,-z,stack-size=1048576 flag to LDFLAGS_NODIST or LINKFORSHARED to increase the thread stack size to 1 MiB.

See also commit 1e9b8f2 (PR gh-134336) which modified Python/ceval.c to no longer use pthread_attr_getstack() on musl.
vstinner commented on Apr 8, 2026
vstinner
on Apr 8, 2026
MemberAuthor
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I tried using $ac_cv_libc variable in configure.ac, but this variable is unset: #143730 (comment).
vstinner commented on May 7, 2026
vstinner
on May 7, 2026
MemberAuthor
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It doesn't seem possible to detect that Python will be linked to musl C library. Maybe another option would be to query the thread stack size to decide to pass -Wl,-z,stack-size=1048576 option to the linker or not.

Example of C program which returns exit code 1 if the thread stack size is smaller than 1 MiB:

#include <pthread.h> #include <assert.h> size_t get(void) { pthread_attr_t attrs; size_t size; int rc = pthread_attr_init(&attrs); assert(rc == 0); rc = pthread_attr_getstacksize(&attrs, &size); assert(rc == 0); return size; } int main() { size_t size = get(); if (size < 1024 * 1024) { return 1; } return 0; }
vstinner commented on May 18, 2026
vstinner
on May 18, 2026
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I wrote PR gh-149993 which adds -Wl,-z,stack-size=1048576 linker flag when Python is linked to the musl C library.
vstinner
added a commit that references this issue on May 20, 2026
gh-148260: Use at least 1 MiB stack size on musl (#149993)
df6c157
vstinner
closed this as completed on May 20, 2026
vstinner
added a commit that references this issue on May 20, 2026
[3.15] gh-148260: Use at least 1 MiB stack size on musl (GH-149993) (#…
c555e26
vstinner commented on May 20, 2026
vstinner
on May 20, 2026
MemberAuthor
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I increased the minimum thread stack size to 1 MiB, whereas musl uses 128 kiB by default. If 1 MiB is not used (glibc uses 8 MiB on Linux!), we can increase the stack size later.
vstinner
added a commit that references this issue on Jun 17, 2026
[3.14] gh-148260: Use at least 1 MiB stack size on musl (GH-149993) (#…
5516672
nvuillam
added a commit that references this issue on Aug 22, 2026
Fix random Segmentation fault crashes on Alpine images (#8733) (#8779)
403219a

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test_threading.test_recursion_limit() does crash (segfault) on Alpine Linux (musl) #148260

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vstinner commented on Apr 8, 2026

vstinner commented on Apr 8, 2026

vstinner commented on Apr 8, 2026

vstinner commented on May 7, 2026

vstinner commented on May 18, 2026

vstinner commented on May 20, 2026

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