Segfault in uvloop 0.22.1 · Issue #706 · MagicStack/uvloop · GitHub
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Segfault in uvloop 0.22.1 #706

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Segfault in uvloop 0.22.1#706

Description

@FHTMitchell
FHTMitchell
opened on Dec 12, 2025

Hi, I see a sefault in uviloop in my fastapi app. I can reproduce it 100% of the time inside the container.

The app is run with

python -X faulthandler -m uvicorn --loop uvloop app:app

If I switch to --loop asyncio I do not get a segfault.

Fatal Python error: Segmentation fault

Thread 0x00007fb2e380f700 (most recent call first):
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/threading.py", line 359 in wait
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/threading.py", line 655 in wait
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/opentelemetry/sdk/_shared_internal/__init__.py", line 159 in worker
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/threading.py", line 1012 in run
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/threading.py", line 1075 in _bootstrap_inner
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/threading.py", line 1032 in _bootstrap

Current thread 0x00007fb74a920400 (most recent call first):
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/middleware/base.py", line 120 in wrap
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/middleware/base.py", line 125 in receive_or_disconnect
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/requests.py", line 228 in stream
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/fastapi/routing.py", line 373 in app
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/fastapi/routing.py", line 106 in app
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42 in wrapped_app
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/fastapi/routing.py", line 120 in app
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/routing.py", line 290 in handle
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/routing.py", line 736 in app
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/routing.py", line 716 in __call__
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/fastapi/middleware/asyncexitstack.py", line 18 in __call__
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42 in wrapped_app
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 63 in __call__
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette_exporter/middleware.py", line 405 in __call__
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/starlette/middleware/base.py", line 144 in coro
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/asyncio/runners.py", line 118 in run
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/asyncio/runners.py", line 195 in run
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/uvicorn/server.py", line 67 in run
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/uvicorn/main.py", line 593 in run
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/uvicorn/main.py", line 423 in main
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/click/core.py", line 824 in invoke
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/click/core.py", line 1269 in invoke
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/click/core.py", line 1406 in main
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/click/core.py", line 1485 in __call__
  File "/opt/app-root/.pixi/envs/default/lib/python3.12/site-packages/uvicorn/__main__.py", line 4 in <module>
  File "<frozen runpy>", line 88 in _run_code
  File "<frozen runpy>", line 198 in _run_module_as_main

Extension modules: uvloop.loop, httptools.parser.parser, httptools.parser.url_parser, websockets.speedups, psutil._psutil_linux, _brotli, zstandard.backend_c, gssapi.raw._enum_extensions.ext_dce, gssapi.raw._enum_extensions.ext_iov_mic, gssapi.raw.oids, gssapi.raw.types, gssapi.raw.cython_converters, gssapi.raw.misc, gssapi.raw.names, gssapi.raw.creds, gssapi.raw.chan_bindings, gssapi.raw.sec_contexts, gssapi.raw.message, gssapi.raw.exceptions, gssapi.raw.ext_s4u, gssapi.raw.ext_cred_store, gssapi.raw.ext_rfc4178, gssapi.raw.ext_rfc5587, gssapi.raw.ext_rfc5588, gssapi.raw.ext_rfc5801, gssapi.raw.ext_cred_imp_exp, gssapi.raw.mech_krb5, gssapi.raw.ext_password, gssapi.raw.ext_password_add, gssapi.raw.ext_dce_aead, gssapi.raw.ext_dce, gssapi.raw.ext_iov_mic, gssapi.raw.ext_krb5, gssapi.raw.ext_rfc6680, gssapi.raw.ext_rfc6680_comp_oid, gssapi.raw.ext_ggf, gssapi.raw.ext_set_cred_opt, _cffi_backend, work._ruamel_yaml, _ruamel_yaml, multidict._multidict, yarl._quoting_c, propcache._helpers_c, aiohttp._http_writer, aiohttp._http_parser, aiohttp._websocket.mask, aiohttp._websocket.reader_c, frozenlist._frozenlist, numpy._core._multiarray_umath, numpy.linalg._umath_linalg, pyarrow.lib, sqlalchemy.cyextension.collections, sqlalchemy.cyextension.immutabledict, sqlalchemy.cyextension.processors, sqlalchemy.cyextension.resultproxy, sqlalchemy.cyextension.util, greenlet._greenlet, pyodbc, numpy.random._common, numpy.random.bit_generator, numpy.random._bounded_integers, numpy.random._pcg64, numpy.random._mt19937, numpy.random._generator, numpy.random._philox, numpy.random._sfc64, numpy.random.mtrand, pandas._libs.tslibs.ccalendar, pandas._libs.tslibs.np_datetime, pandas._libs.tslibs.dtypes, pandas._libs.tslibs.base, pandas._libs.tslibs.nattype, pandas._libs.tslibs.timezones, pandas._libs.tslibs.fields, pandas._libs.tslibs.timedeltas, pandas._libs.tslibs.tzconversion, pandas._libs.tslibs.timestamps, pandas._libs.properties, pandas._libs.tslibs.offsets, pandas._libs.tslibs.strptime, pandas._libs.tslibs.parsing, pandas._libs.tslibs.conversion, pandas._libs.tslibs.period, pandas._libs.tslibs.vectorized, pandas._libs.ops_dispatch, pandas._libs.missing, pandas._libs.hashtable, pandas._libs.algos, pandas._libs.interval, pandas._libs.lib, pyarrow._compute, pandas._libs.ops, pandas._libs.hashing, pandas._libs.arrays, pandas._libs.tslib, pandas._libs.sparse, pandas._libs.internals, pandas._libs.indexing, pandas._libs.index, pandas._libs.writers, pandas._libs.join, pandas._libs.window.aggregations, pandas._libs.window.indexers, pandas._libs.reshape, pandas._libs.groupby, pandas._libs.json, pandas._libs.parsers, pandas._libs.testing, pyarrow._feather, _cyutility, scipy._cyutility, scipy._lib._ccallback_c, scipy.sparse._sparsetools, _csparsetools, scipy.sparse._csparsetools, google._upb._message, confluent_kafka.cimpl (total: 117)
Segmentation fault (core dumped)

Environment (abridged)

arrow_odbc      7.0.4    py312h4413252_1 
fastapi         0.124.2  hc0e8453_0         
orjson          3.11.4   py312h868fb18_1     
pyodbc          5.3.0    py312h1289d80_0   
pytest          8.4.2    pyhcf101f3_1        
pytest-asyncio  0.25.3   pyh29332c3_0        
python          3.12.12  hd63d673_1_cpython 
simdutf         5.4.15   h5888daf_0        
sqlalchemy      2.0.45   py312h4c3975b_0
uvicorn         0.38.0   pyh31011fe_0       
uvloop          0.22.1   py312h4c3975b_1

I wasn't able to get a coredump because I don't have access but could possibly get one if required.

Activity

  1. FHTMitchell
    changed the title [-]Segfault in uvloop 22.1[/-] [+]Segfault in uvloop 0.22.1[/+] on Dec 12, 2025

  2. fantix commented on Dec 22, 2025

    @fantix
    fantix
    on Dec 22, 2025
    Member

    coredump would be nice to have! Thanks for your report! Also:

    • What is your app.py look like? Just a plain FastAPI app?
    • By "container" you mean a pixi environment, or actual containers like Docker? Would be great if you can give me a command to reproduce

  3. KowalskiThomas commented on Apr 17, 2026

    @KowalskiThomas
    KowalskiThomas
    on Apr 17, 2026

    👋 Hello!

    For your information, we are using uvloop and seeing similar crashes in some apps. I can't share the code, but I do have a native stack trace, which I figured might be worth sharing. (This is on Python 3.12.)

    Error UnixSignal: Process terminated with SI_KERNEL (SIGSEGV)
#0   0x000075d5a4383921 __pyx_f_6uvloop_4loop___uv_close_handle_cb (/project/uvloop/loop.c:85306:20)
#1   0x000075d5a443e711 uv__finish_close (/project/build/libuv-x86_64/src/unix/core.c:351:5)
#2   0x000075d5a443e711 uv__run_closing_handles (/project/build/libuv-x86_64/src/unix/core.c:365:5)
#3   0x000075d5a443e711 uv_run (/project/build/libuv-x86_64/src/unix/core.c:463:5)
#4   0x000075d5a435fdb5 __pyx_f_6uvloop_4loop_4Loop__Loop__run (/project/uvloop/loop.c:18458:23)
#5   0x000075d5a43c7e50 __pyx_f_6uvloop_4loop_4Loop__run (/project/uvloop/loop.c:18876:18)
#6   0x000075d5a43d8cf0 __pyx_pf_6uvloop_4loop_4Loop_24run_forever (/project/uvloop/loop.c:31528:18)
#7   0x000075d5a43d8cf0 __pyx_pw_6uvloop_4loop_4Loop_25run_forever (/project/uvloop/loop.c:31331:13)
#8   0x00005fed9aa8a59c _PyObject_VectorcallTstate (/usr/src/python/./Include/internal/pycore_call.h:92:11)
#9   0x00005fed9aa8a59c PyObject_VectorcallMethod (/usr/src/python/Objects/call.c:887:24)
#10  0x000075d5a43dcd60 __pyx_pf_6uvloop_4loop_4Loop_44run_until_complete (/project/uvloop/loop.c:33768:23)
#11  0x000075d5a43de591 __pyx_pw_6uvloop_4loop_4Loop_45run_until_complete (/project/uvloop/loop.c:33318:13)
#12  0x00005fed9aa89a18 _PyObject_VectorcallTstate (/usr/src/python/./Include/internal/pycore_call.h:92:11)
#13  0x00005fed9aa89a18 PyObject_Vectorcall (/usr/src/python/Objects/call.c:325:12)
#14  0x00005fed9aab0807 _PyEval_EvalFrameDefault (/usr/src/python/Python/bytecodes.c:2715:19)
#15  0x00005fed9aa8cba6 _PyEval_EvalFrame (/usr/src/python/./Include/internal/pycore_ceval.h:89:16)
#16  0x00005fed9aa8cba6 gen_send_ex2 (/usr/src/python/Objects/genobject.c:230:14)
#17  0x000075d5a747adc7 task_step_impl (/usr/src/python/./Modules/_asynciomodule.c:2869:22)
#18  0x000075d5a747b5a2 task_step (/usr/src/python/./Modules/_asynciomodule.c:3188:11)
#19  0x00005fed9aa896fe _PyObject_MakeTpCall (/usr/src/python/Objects/call.c:240:18)
#20  0x00005fed9aa0c80c _PyObject_VectorcallTstate (/usr/src/python/./Include/internal/pycore_call.h:90:16)
#21  0x00005fed9aa0c80c context_run (/usr/src/python/Python/context.c:668:29)
#22  0x00005fed9aafbd7b cfunction_vectorcall_FASTCALL_KEYWORDS (/usr/src/python/Objects/methodobject.c:438:24)
#23  0x00005fed9aab41a9 _PyEval_EvalFrameDefault (/usr/src/python/Python/bytecodes.c:3263:26)
#24  0x00005fed9ab334b9 PyEval_EvalCode (/usr/src/python/Python/ceval.c:578:21)
#25  0x00005fed9ab5152c run_eval_code_obj (/usr/src/python/Python/pythonrun.c:1722:9)
#26  0x00005fed9ab514a4 run_mod (/usr/src/python/Python/pythonrun.c:1743:19)
#27  0x00005fed9ab51061 pyrun_file (/usr/src/python/Python/pythonrun.c:1643:15)
#28  0x00005fed9ab50ea7 _PyRun_SimpleFileObject (/usr/src/python/Python/pythonrun.c:433:13)
#29  0x00005fed9ab50cc7 _PyRun_AnyFileObject (/usr/src/python/Python/pythonrun.c:78:15)
#30  0x00005fed9ab5b230 pymain_run_file_obj (/usr/src/python/Modules/main.c:360:15)
#31  0x00005fed9ab5b230 pymain_run_file (/usr/src/python/Modules/main.c:379:15)
#32  0x00005fed9ab5b230 pymain_run_python (/usr/src/python/Modules/main.c:633:21)
#33  0x00005fed9ab5b230 Py_RunMain (/usr/src/python/Modules/main.c:713:5)
#34  0x00005fed9ab5adbd Py_BytesMain (/usr/src/python/Modules/main.c:767:12)
#35  0x000075d5a8000e40 __libc_start_main
#36  0x00005fed9aad32d5 _start

    LMK if there's anything else I can help with or provide!

  4. cwagner22
    added a commit that references this issue on Sep 3, 2026

    fix: run uvicorn on stock asyncio loop to avoid uvloop 0.22.1 segfault

    867c45f
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