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GeezerVison is showing github.com — bpo-46070: Revert "bpo-36854: Move _PyRuntimeState.gc to PyInterpreterState (GH-17287)" by vstinner · Pull Request #30564 · python/cpython

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@vstinner

@vstinner vstinner commented Jan 12, 2022 •
edited by bedevere-bot
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This reverts commit 7247407.

Fix a random crash involving subinterpreters on Windows. Revert the
change which made the gc module state per interpreter: the gc module
state is shared again by all interpreters.

https://bugs.python.org/issue36854

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@vstinner
vstinner requested review from iritkatriel and pablogsal as code owners January 12, 2022 22:36
@bedevere-bot bedevere-bot added the awaiting core review label Jan 12, 2022
@vstinner
vstinner marked this pull request as draft January 12, 2022 23:23
@vstinner
bpo-46070: Revert "bpo-36854: Move _PyRuntimeState.gc to PyInterprete…
b0bb111 
…rState (GH-17287)"

This reverts commit 7247407.

Fix a random crash involving subinterpreters on Windows. Revert the
change which made the gc module state per interpreter: the gc module
state is shared again by all interpreters.
@vstinner vstinner changed the title Revert "bpo-36854: Move _PyRuntimeState.gc to PyInterpreterState (GH-17287)" bpo-46070: Revert "bpo-36854: Move _PyRuntimeState.gc to PyInterpreterState (GH-17287)" Jan 12, 2022
ericsnowcurrently
ericsnowcurrently approved these changes Jan 12, 2022
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LGTM

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@bedevere-bot bedevere-bot added awaiting merge and removed awaiting core review labels Jan 12, 2022
@ericsnowcurrently

ericsnowcurrently commented Jan 12, 2022

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FTR, this is to resolve https://bugs.python.org/issue46070.
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@ericsnowcurrently

ericsnowcurrently commented Jan 12, 2022

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BTW, do we have any sort of reproducer for which we can add a test? That way we don't cause the same problem when we move GC back to PyInterpreterState later.
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@vstinner

vstinner commented Jan 12, 2022

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BTW, do we have any sort of reproducer for which we can add a test?

So far, I was only able to reproduce the crash on Windows with Python 3.9 using win_py399_crash_reproducer.py script of https://bugs.python.org/issue46070 I failed to write a reliable reproducer which works on any platform.

I didn't fully understand the root issue. My theory is that an object is shared by two interprereter. The first interpreter runs a GC collection which traverses this shared object, an object finalizer releases the GIL. Now the second interpreter gets the GIL and also runs a GC collection which traverses the same shared object.

The problem is that the PyGC_Head header cannot be modified by two interpreters in parallel: data races are likely and Python does crash somehow.

Since the bug is a crash and so far nobody managed to fully understand it, IMO it's safer to revert the change for now.
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@ericsnowcurrently

ericsnowcurrently commented Jan 13, 2022

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Your theory sounds plausible.
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@vstinner

vstinner commented Jan 13, 2022

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I wrote a simpler fix: GH-30577.
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@vstinner vstinner closed this Jan 13, 2022
@vstinner
vstinner deleted the revert_gc branch January 13, 2022 18:31
@graysky graysky mannequin mentioned this pull request Apr 10, 2022
[subinterpreters] crash when importing _sre in subinterpreters in parallel (Python 3.9 regression) #90228
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@ericsnowcurrently ericsnowcurrently ericsnowcurrently approved these changes
@iritkatriel iritkatriel Awaiting requested review from iritkatriel
@pablogsal pablogsal Awaiting requested review from pablogsal pablogsal is a code owner
@ZeroIntensity ZeroIntensity Awaiting requested review from ZeroIntensity ZeroIntensity will be requested when the pull request is marked ready for review ZeroIntensity is a code owner
@FFY00 FFY00 Awaiting requested review from FFY00 FFY00 will be requested when the pull request is marked ready for review FFY00 is a code owner

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@vstinner @ericsnowcurrently @bedevere-bot
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