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GeezerVison is showing github.com — gh-135871: Fix needless spinning in _PyMutex_LockTimed (timeout==0) by jtibbertsma · Pull Request #135872 · python/cpython

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jtibbertsma commented Jun 24, 2025 •

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Move the timeout == 0 guard outside the else so a non-blocking call returns immediately after a failed CAS instead of entering the spin loop.
~~Reload v on every spin iteration, allowing timed/blocking callers to notice an unlock promptly.~~

No-GIL builds now honor the semantics of non-blocking attempts and avoid wasted CPU; GIL builds are unaffected (MAX_SPIN_COUNT == 0).

Issue: Non-blocking _PyMutex_LockTimed spins and may fail unnecessarily in no-GIL builds #135871

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python-cla-bot Bot commented Jun 24, 2025 •

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bedevere-app Bot added the awaiting review label

Jun 24, 2025

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Jun 24, 2025

Non-blocking _PyMutex_LockTimed spins and may fail unnecessarily in no-GIL builds #135871

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jtibbertsma force-pushed the fix-issue-135871 branch from 94f04f5 to 88e8a1c Compare

June 24, 2025 01:00

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jtibbertsma force-pushed the fix-issue-135871 branch from 88e8a1c to 45811fd Compare

June 24, 2025 01:01

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          pythongh-135871: Fix needless spinning in _PyMutex_LockTimed (timeout…

7296a7d

…==0)

* Move the timeout == 0 guard outside the else so a non-blocking call
  returns immediately after a failed CAS instead of entering the spin loop.
* Reload v on every spin iteration, allowing timed/blocking callers to
  notice an unlock promptly.

No-GIL builds now honor the semantics of non-blocking attempts and avoid
wasted CPU; GIL builds are unaffected (MAX_SPIN_COUNT == 0).

jtibbertsma force-pushed the fix-issue-135871 branch from 45811fd to 7296a7d Compare

June 24, 2025 01:09

ZeroIntensity reviewed

Jun 24, 2025

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Please don't force-push, we squash at the end.

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Misc/NEWS.d/next/Core and Builtins/2025-06-23-18-08-32.gh-issue-135871.50C528.rst Resolved

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ZeroIntensity requested review from colesbury and pitrou

June 24, 2025 01:47

ZeroIntensity added topic-free-threading needs backport to 3.14 bugs and security fixes labels

Jun 24, 2025

jtibbertsma added 2 commits

June 23, 2025 18:54


          Update news entry

4b5123c


          Fix filename

8a4e9ad

ZeroIntensity removed the request for review from pitrou

June 24, 2025 01:58

ZeroIntensity commented Jun 24, 2025

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(Sorry Antoine, hit the wrong button!)

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vstinner reviewed

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Misc/NEWS.d/next/Core_and_Builtins/2025-06-23-18-08-32.gh-issue-135871.50C528.rst Resolved

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colesbury reviewed

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Python/lock.c Outdated

		// Spin for a bit.
		_Py_yield();
		spin_count++;
		v = _Py_atomic_load_uint8_relaxed(&m->_bits);

colesbury Jun 24, 2025

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This looks like it fixes a logic bug but it also hurts performance

See Tools/lockbench/lockbench.py

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jtibbertsma Jun 25, 2025

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thanks for the review! I'm convinced, this line reduces throughput on my machine by something like 40% for 8 threads. I think changing else if to if still makes sense though. I updated the PR.

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          Keep timeout==0 fast-fail, drop spin-loop state reload

3fb8f37

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LGTM

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bedevere-app Bot added awaiting merge and removed awaiting review labels

Jun 25, 2025

vstinner added the needs backport to 3.13 only security fixes label

Jun 25, 2025

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LGTM

Let's investigate (and fix) the missing load separately

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Misc/NEWS.d/next/Core_and_Builtins/2025-06-23-18-08-32.gh-issue-135871.50C528.rst Resolved

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          Update Misc/NEWS.d/next/Core_and_Builtins/2025-06-23-18-08-32.gh-issu…

8cbb992

…e-135871.50C528.rst

colesbury enabled auto-merge (squash)

June 25, 2025 16:15

colesbury merged commit cbfaf41 into python:main

Jun 25, 2025

miss-islington-app Bot commented Jun 25, 2025

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Thanks @jtibbertsma for the PR, and @colesbury for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

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bedevere-app Bot removed the awaiting merge label

Jun 25, 2025

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request

Jun 25, 2025


          Fix needless spinning in _PyMutex_LockTimed with zero timeout (pyth…

535b8d6

…ongh-135872)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.
(cherry picked from commit cbfaf41)

Co-authored-by: Joseph Tibbertsma <[email protected]>

miss-islington-app Bot commented Jun 25, 2025

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Sorry, @jtibbertsma and @colesbury, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker cbfaf41caf135b8598a560854cd59e992a2ccfed 3.13

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bedevere-app Bot commented Jun 25, 2025

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GH-135946 is a backport of this pull request to the 3.14 branch.

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miss-islington-app Bot assigned colesbury

Jun 25, 2025

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label

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bedevere-app Bot commented Jun 25, 2025

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GH-135946 is a backport of this pull request to the 3.14 branch.

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colesbury pushed a commit to colesbury/cpython that referenced this pull request

Jun 25, 2025


          [3.13] Fix needless spinning in _PyMutex_LockTimed with zero timeout (

697bdf5

pythongh-135872)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.
(cherry picked from commit cbfaf41)

Co-authored-by: Joseph Tibbertsma <[email protected]>

bedevere-app Bot commented Jun 25, 2025

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GH-135947 is a backport of this pull request to the 3.13 branch.

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bedevere-app Bot removed the needs backport to 3.13 only security fixes label

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bedevere-app Bot commented Jun 25, 2025

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GH-135947 is a backport of this pull request to the 3.13 branch.

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colesbury pushed a commit that referenced this pull request

Jun 25, 2025


          [3.14] gh-135871: Fix needless spinning in _PyMutex_LockTimed with …

80fc62f

…zero timeout (gh-135872) (gh-135946)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.
(cherry picked from commit cbfaf41)

Co-authored-by: Joseph Tibbertsma <[email protected]>

colesbury added a commit that referenced this pull request

Jun 25, 2025


          [3.13] gh-135871: Fix needless spinning in _PyMutex_LockTimed with …

c64df2a

…zero timeout (gh-135872) (gh-135947)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.
(cherry picked from commit cbfaf41)

Co-authored-by: Joseph Tibbertsma <[email protected]>

AndPuQing pushed a commit to AndPuQing/cpython that referenced this pull request

Jul 11, 2025


          Fix needless spinning in _PyMutex_LockTimed with zero timeout (pyth…

b7de590

…ongh-135872)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.

Pranjal095 pushed a commit to Pranjal095/cpython that referenced this pull request

Jul 12, 2025


          Fix needless spinning in _PyMutex_LockTimed with zero timeout (pyth…

716ea1b

…ongh-135872)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.

taegyunkim pushed a commit to taegyunkim/cpython that referenced this pull request

Aug 4, 2025


          Fix needless spinning in _PyMutex_LockTimed with zero timeout (pyth…

dcca67b

…ongh-135872)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.

Agent-Hellboy pushed a commit to Agent-Hellboy/cpython that referenced this pull request

Aug 19, 2025


          Fix needless spinning in _PyMutex_LockTimed with zero timeout (pyth…

c24fa3a

…ongh-135872)

The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.

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jtibbertsma commented Jun 24, 2025 •

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