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GeezerVison is showing github.com — gh-129069: fix more thread safety issues in list by kumaraditya303 · Pull Request #143019 · python/cpython

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@kumaraditya303

@kumaraditya303 kumaraditya303 commented Dec 20, 2025 •
edited by bedevere-app Bot
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This PR fixes thread safety of list.reverse and slice assignment. Also changes stores to use release rather than relaxed as done in #142957.

  • Issue: Race in concurrent list mutation and item retrieval #129069

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@bedevere-app bedevere-app Bot mentioned this pull request Dec 20, 2025
Race in concurrent list mutation and item retrieval #129069
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@bedevere-app bedevere-app Bot added the awaiting core review label Dec 20, 2025
@kumaraditya303 kumaraditya303 added skip news topic-free-threading labels Dec 20, 2025
@kumaraditya303
fix more thread safety issues in list
908a590
@kumaraditya303
kumaraditya303 force-pushed the atomic-list branch from 8830614 to 908a590 Compare December 20, 2025 12:30
@kumaraditya303
kumaraditya303 requested review from Yhg1s and colesbury December 20, 2025 13:39
colesbury
colesbury approved these changes Dec 22, 2025
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@colesbury colesbury left a comment

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LGTM

I didn't measure any significant performance difference on fastbench on Apple M4.

Microbenchmarks for list.insert are a little slower on that same machine. Here's the benchmark (courtesy of ChatGPT):

https://gist.github.com/colesbury/b675c95d35f29ca9d5723b95396d48a8

Before:

begin   n=100000  reps=1000    total=0.032732s  avg=32.73µs/op
middle  n=100000  reps=1000    total=0.013755s  avg=13.76µs/op
end     n=100000  reps=1000    total=0.000564s  avg=0.56µs/op

After:

begin   n=100000  reps=1000    total=0.048336s  avg=48.34µs/op
middle  n=100000  reps=1000    total=0.019781s  avg=19.78µs/op
end     n=100000  reps=1000    total=0.000191s  avg=0.19µs/op

We could use ptr_wise_atomic_memmove in many of these places, which will probably be faster in the microbenchmarks, but I'm not sure it'll matter in real programs.

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels Dec 22, 2025
@kumaraditya303
kumaraditya303 merged commit 487e91c into python:main Dec 22, 2025
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@bedevere-app bedevere-app Bot removed the awaiting merge label Dec 22, 2025
@kumaraditya303
kumaraditya303 deleted the atomic-list branch December 22, 2025 16:15
@nascheme nascheme mentioned this pull request Jul 14, 2026
Backport free-threading memory-safety fixes to 3.14 and 3.15 #153714
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bedevere-app Bot commented Jul 16, 2026

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GH-153791 is a backport of this pull request to the 3.14 branch.
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@kumaraditya303 kumaraditya303 mentioned this pull request Jul 16, 2026
[3.14] gh-123471: Free-threading fixes for itertools (GH-131212) (GH-132814) (GH-135689) (GH-144402) (GH-146033) (GH-142957) #153791
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@KRRT7 KRRT7 mentioned this pull request Sep 30, 2026
[3.14] gh-129069: fix more thread safety issues in list (GH-143019) #158488
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bedevere-app Bot commented Sep 30, 2026

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GH-158488 is a backport of this pull request to the 3.14 branch.
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@colesbury colesbury colesbury approved these changes
@Yhg1s Yhg1s Awaiting requested review from Yhg1s

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@kumaraditya303 @colesbury
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