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GeezerVison is showing github.com — gh-150411: fix `gc_generation.count` race by LindaSummer · Pull Request #150413 · python/cpython

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@LindaSummer

@LindaSummer LindaSummer commented May 25, 2026

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Issue

gh-150411

Root Cause

Refer the analytics in issue gh-150411, it should be a gc_generation.count update during the cyclic object allocation triggered the local allocation count migrated to young generation. Meantime we try to read the gc_generation.count without sync caused the race.

cpython/Python/gc_free_threading.c

Lines 2017 to 2037 in c714b56

static void
record_allocation(PyThreadState *tstate)
{
struct _gc_thread_state *gc = &((_PyThreadStateImpl *)tstate)->gc;
// We buffer the allocation count to avoid the overhead of atomic
// operations for every allocation.
gc->alloc_count++;
if (gc->alloc_count >= LOCAL_ALLOC_COUNT_THRESHOLD) {
// TODO: Use Py_ssize_t for the generation count.
GCState *gcstate = &tstate->interp->gc;
_Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
gc->alloc_count = 0;
if (gc_should_collect(gcstate) &&
!_Py_atomic_load_int_relaxed(&gcstate->collecting))
{
_Py_ScheduleGC(tstate);
}
}
}

I find this problem during proposing gh-150356. So they have similar reproduce way.

Proposed Changes

I added an atomic relax load guard for the gc_generation.count.
It was protected in other places expect current one.

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@LindaSummer
LindaSummer requested a review from pablogsal as a code owner May 25, 2026 16:18
@bedevere-app bedevere-app Bot mentioned this pull request May 25, 2026
Data race between record_allocation and gc_get_count_impl #150411
Closed
@bedevere-app bedevere-app Bot added the awaiting review label May 25, 2026
@pablogsal

pablogsal commented May 25, 2026

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Same concerns as #150356 (comment)

CC @nascheme
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kumaraditya303
kumaraditya303 reviewed Jun 4, 2026
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Comment thread Modules/gcmodule.c Outdated
gcstate->old[0].count,
gcstate->old[1].count);
_Py_atomic_load_int_relaxed(&gcstate->young.count),
_Py_atomic_load_int_relaxed(&gcstate->old[0].count),

@kumaraditya303 kumaraditya303 Jun 4, 2026

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Does gcstate->old[0].count need to be atomic here? I think it is only ever written under stw so it is unnecessary.

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@LindaSummer LindaSummer Jun 4, 2026

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Hi @kumaraditya303 ,

Thanks very much for your review! ❤️
Yes, I searched the usage of count and find that only the young.count is updated without STW guard.
old[generation].count are all protected by STW.
So we only need to make young.count use atomic action and old[generation].count read is safe.

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LindaSummer added 3 commits June 4, 2026 23:33
@LindaSummer
fix gc_generation.count race
c916f01
@LindaSummer
add blurb
b2e9bca
@LindaSummer
remove useless atomic
639cc24
@LindaSummer
LindaSummer force-pushed the gc_count_tsan branch from b71383e to 639cc24 Compare June 4, 2026 15:50
@LindaSummer

LindaSummer commented Jun 4, 2026

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Hi @pablogsal and @kumaraditya303 ,

Thanks very much for your review and suggestions!

I have updated the patch with only atomic for young.count.
The old[generation].count write is protected by STW.
The young.count is a frequently updated counter and in this patch we only read its snapshot.
So I think the result is consistent since we only synced one variable.

Please correct me if I made any mistake or misunderstanding.

Wish you a good day! 🌞
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kumaraditya303
kumaraditya303 reviewed Jun 6, 2026
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Comment thread Lib/test/test_free_threading/test_gc.py Resolved

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@kumaraditya303
Update Lib/test/test_free_threading/test_gc.py
4ab7a60
kumaraditya303
kumaraditya303 approved these changes Jun 6, 2026
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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting review labels Jun 6, 2026
@kumaraditya303
kumaraditya303 enabled auto-merge (squash) June 6, 2026 16:36
@kumaraditya303
kumaraditya303 merged commit 12af26d into python:main Jun 6, 2026
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@bedevere-app bedevere-app Bot removed the awaiting merge label Jun 6, 2026
philthompson10 pushed a commit to philthompson10/cpython that referenced this pull request Jun 17, 2026
@LindaSummer @philthompson10
pythongh-150411: fix gc_generation.count race in free-threading (py…
fa2eb1e 
…thon#150413)
@nascheme nascheme mentioned this pull request Jul 14, 2026
Backport free-threading memory-safety fixes to 3.14 and 3.15 #153714
Closed
@nascheme nascheme added needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Jul 16, 2026
@miss-islington-app

miss-islington-app Bot commented Jul 16, 2026

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Thanks @LindaSummer for the PR, and @kumaraditya303 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖
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@miss-islington-app

miss-islington-app Bot commented Jul 16, 2026

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Thanks @LindaSummer for the PR, and @kumaraditya303 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖
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@nascheme nascheme added the topic-free-threading label Jul 16, 2026
@bedevere-app

bedevere-app Bot commented Jul 16, 2026

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GH-153787 is a backport of this pull request to the 3.14 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Jul 16, 2026
@bedevere-app

bedevere-app Bot commented Jul 16, 2026

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GH-153788 is a backport of this pull request to the 3.15 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Jul 16, 2026
kumaraditya303 pushed a commit that referenced this pull request Jul 16, 2026
@miss-islington @LindaSummer
[3.15] gh-150411: fix gc_generation.count race in free-threading (G…
6d6f526 
…H-150413) (#153788)

gh-150411: fix `gc_generation.count` race in free-threading (GH-150413)
(cherry picked from commit 12af26d)

Co-authored-by: Edward Xu <[email protected]>
nascheme pushed a commit that referenced this pull request Jul 17, 2026
@miss-islington @LindaSummer
[3.14] gh-150411: fix gc_generation.count race in free-threading (G…
46dbee9 
…H-150413) (GH-153787)

(cherry picked from commit 12af26d)

Co-authored-by: Edward Xu <[email protected]>
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@kumaraditya303 kumaraditya303 kumaraditya303 approved these changes
@pablogsal pablogsal Awaiting requested review from pablogsal pablogsal is a code owner

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4 participants

@LindaSummer @pablogsal @kumaraditya303 @nascheme
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