GH-149142: Fix decimal `mpd_context_t.status` race in free-threading by LindaSummer · Pull Request #150598 · python/cpython

LindaSummer · 2026-05-30T06:35:53Z

Issue

Proposed Changes

Add mutex in free-threading build to guard the PyDecContextObject.ctx.status.
It solves the race in concurrent dec_addstatus and clear_flags.

Comment

For other flags like traps are not guarded in this PR.
I plan to update in later PRs with new test cases.

LindaSummer · 2026-05-30T07:10:29Z

During this patch, I find another race in decimal.DefaultContext.

In context_new, we may create a new context from decimal.DefaultContext.
But the decimal.DefaultContext could be updated in another thread.

cpython/Modules/_decimal/_decimal.c

Lines 1467 to 1469 in 1c7011d

    
           if (state->default_context_template) { 
        
               *ctx = *CTX(state->default_context_template); 
        
           }

So I think that we should have a way to get a snapshot of default_context_template just like the contextvar's HMAT in #146482.

Refer to our documentation we could see below:

This context is most useful in multi-threaded environments. Changing one of the fields before threads are started has the effect of setting system-wide defaults. Changing the fields after threads have started is not recommended as it would require thread synchronization to prevent race conditions.

I'm not sure if this case is valid refer to our documentation.
But even it is not suggested, I think that the TSAN may still need to be fixed.

If we plan to fix this one, I am appreciating to create a new issue and try to solve it.

Here is the demo reproduce code and TSAN report.

import decimal
import threading


N_THREADS = 8
ITERATIONS = 100_000


def run_concurrently(workers):
    barrier = threading.Barrier(len(workers))
    threads = [threading.Thread(target=worker, args=(barrier,))
               for worker in workers]
    for thread in threads:
        thread.start()
    for thread in threads:
        thread.join()


default_context = decimal.DefaultContext

def creator(barrier):
    barrier.wait()
    for _ in range(ITERATIONS):
        decimal.Context()

def mutator(barrier):
    barrier.wait()
    for i in range(ITERATIONS):
        default_context.prec = 9 + (i % 10)
        default_context.Emax = 999999 - (i % 10)
        default_context.Emin = -999999 + (i % 10)
        default_context.create_decimal("1.23456")
        default_context.clear_flags()

run_concurrently([creator] * N_THREADS + [mutator] * N_THREADS)

TSAN report.

SUMMARY: ThreadSanitizer: data race Modules/_decimal/_decimal.c:616 in dec_addstatus
==================
==================
WARNING: ThreadSanitizer: data race (pid=1872326)
  Write of size 4 at 0x7f21063640bc by thread T16:
    #0 dec_addstatus Modules/_decimal/_decimal.c:616 (_decimal.cpython-316t-x86_64-linux-gnu.so+0xf475)
    #1 PyDecType_FromCString Modules/_decimal/_decimal.c:2327 (_decimal.cpython-316t-x86_64-linux-gnu.so+0x13106)
    #2 PyDecType_FromSequence Modules/_decimal/_decimal.c:2934 (_decimal.cpython-316t-x86_64-linux-gnu.so+0x13106)
    #3 PyDec_FromObject Modules/_decimal/_decimal.c:3234 (_decimal.cpython-316t-x86_64-linux-gnu.so+0x13106)
    #4 _decimal_Context_create_decimal_impl Modules/_decimal/_decimal.c:3292 (_decimal.cpython-316t-x86_64-linux-gnu.so+0x13106)
    #5 _decimal_Context_create_decimal Modules/_decimal/clinic/_decimal.c.h:853 (_decimal.cpython-316t-x86_64-linux-gnu.so+0x13106)
    #6 _Py_BuiltinCallFast_StackRef Python/ceval.c:815 (python3.16+0x3bfa1b)
    #7 _PyEval_EvalFrameDefault Python/generated_cases.c.h:2420 (python3.16+0x3df9d5)
    #8 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ec7d7)
    #9 _PyEval_Vector Python/ceval.c:2134 (python3.16+0x3ec7d7)
    #10 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152110)
    #11 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15370f)
    #12 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15370f)
    #13 method_vectorcall Objects/classobject.c:55 (python3.16+0x1585b6)
    #14 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x42729b)
    #15 context_run Python/context.c:728 (python3.16+0x42729b)
    #16 method_vectorcall_FASTCALL_KEYWORDS Objects/descrobject.c:421 (python3.16+0x1743fe)
    #17 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151cfc)
    #18 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151cfc)
    #19 _Py_VectorCallInstrumentation_StackRefSteal Python/ceval.c:766 (python3.16+0x3bf555)
    #20 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1846 (python3.16+0x3cf120)
    #21 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ec7d7)
    #22 _PyEval_Vector Python/ceval.c:2134 (python3.16+0x3ec7d7)
    #23 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152110)
    #24 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15370f)
    #25 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15370f)
    #26 method_vectorcall Objects/classobject.c:55 (python3.16+0x1585b6)
    #27 _PyVectorcall_Call Objects/call.c:273 (python3.16+0x154be8)
    #28 _PyObject_Call Objects/call.c:348 (python3.16+0x154fe1)
    #29 PyObject_Call Objects/call.c:373 (python3.16+0x154fe1)
    #30 thread_run Modules/_threadmodule.c:388 (python3.16+0x608d30)
    #31 pythread_wrapper Python/thread_pthread.h:234 (python3.16+0x5072d2)
    #32 <null> <null> (libtsan.so.2+0x4cf1a)

  Previous read of size 8 at 0x7f21063640b8 by thread T1:
    #0 context_new Modules/_decimal/_decimal.c:1468 (_decimal.cpython-316t-x86_64-linux-gnu.so+0xd6af)
    #1 type_call Objects/typeobject.c:2467 (python3.16+0x28839b)
    #2 _PyObject_MakeTpCall Objects/call.c:242 (python3.16+0x150dae)
    #3 _PyObject_VectorcallTstate Include/internal/pycore_call.h:142 (python3.16+0x151d61)
    #4 _PyObject_VectorcallTstate Include/internal/pycore_call.h:129 (python3.16+0x151d61)
    #5 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d61)
    #6 _Py_VectorCall_StackRefSteal Python/ceval.c:724 (python3.16+0x3bf125)
    #7 _PyEval_EvalFrameDefault Python/generated_cases.c.h:4362 (python3.16+0x3db2fc)
    #8 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ec7d7)
    #9 _PyEval_Vector Python/ceval.c:2134 (python3.16+0x3ec7d7)
    #10 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152110)
    #11 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15370f)
    #12 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15370f)
    #13 method_vectorcall Objects/classobject.c:55 (python3.16+0x1585b6)
    #14 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x42729b)
    #15 context_run Python/context.c:728 (python3.16+0x42729b)
    #16 method_vectorcall_FASTCALL_KEYWORDS Objects/descrobject.c:421 (python3.16+0x1743fe)
    #17 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151cfc)
    #18 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151cfc)
    #19 _Py_VectorCallInstrumentation_StackRefSteal Python/ceval.c:766 (python3.16+0x3bf555)
    #20 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1846 (python3.16+0x3cf120)
    #21 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ec7d7)
    #22 _PyEval_Vector Python/ceval.c:2134 (python3.16+0x3ec7d7)
    #23 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152110)
    #24 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15370f)
    #25 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15370f)
    #26 method_vectorcall Objects/classobject.c:55 (python3.16+0x1585b6)
    #27 _PyVectorcall_Call Objects/call.c:273 (python3.16+0x154be8)
    #28 _PyObject_Call Objects/call.c:348 (python3.16+0x154fe1)
    #29 PyObject_Call Objects/call.c:373 (python3.16+0x154fe1)
    #30 thread_run Modules/_threadmodule.c:388 (python3.16+0x608d30)
    #31 pythread_wrapper Python/thread_pthread.h:234 (python3.16+0x5072d2)
    #32 <null> <null> (libtsan.so.2+0x4cf1a)

LindaSummer · 2026-05-30T07:41:31Z

Hi @devdanzin ,

Could you help take a review?
Please correct me if I made any mistake or misunderstanding.
Wish you a good day! ❤

skirpichev · 2026-06-01T01:50:39Z

Refer to our documentation we could see below

You refer to the wrong place in docs. They have a dedicated sections, that says:

To control the defaults so that each thread will use the same values throughout the application, directly modify the DefaultContext object. This should be done before any threads are started so that there won’t be a race condition between threads calling getcontext().

BTW, I'm not sure that referenced issue is valid.

LindaSummer · 2026-06-02T03:13:41Z

Refer to our documentation we could see below

You refer to the wrong place in docs. They have a dedicated sections, that says:

To control the defaults so that each thread will use the same values throughout the application, directly modify the DefaultContext object. This should be done before any threads are started so that there won’t be a race condition between threads calling getcontext().

BTW, I'm not sure that referenced issue is valid.

Hi @skirpichev ,

Thanks very much for your time and correction!

Got it! The doc has description of this race condition case, I wouldn't make it a new issue except it breaks the contract.

By the way, could I ask for your your review and give me some suggestions on this patch?

Wish you a good day!🌞

skirpichev · 2026-06-02T03:49:13Z

could I ask for your your review and give me some suggestions on this patch?

I'm not sure that referenced issue is valid. Perhaps, we should change docs instead.

LindaSummer added 4 commits May 30, 2026 00:10

fix: fix decimal.Context.status race in free-threading

31bbc40

fix comment changes

71397b2

fix wrong lock

012b81f

add guard for free threading

5676be4

bedevere-app Bot added the awaiting review label May 30, 2026

bedevere-app Bot mentioned this pull request May 30, 2026

_decimal: mpd_context_t::status/traps mutated non-atomically leading to data race #149142

Open

revert useless change

64d4fcb

Merge branch 'main' into fix/decimal_ctx_status

d598e3c

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GH-149142: Fix decimal `mpd_context_t.status` race in free-threading#150598

GH-149142: Fix decimal `mpd_context_t.status` race in free-threading#150598
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LindaSummer:fix/decimal_ctx_status

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