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bpo-39776: Lock ++interp->tstate_next_unique_id. (GH-18746) #18746

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Merged
merged 2 commits into from
Mar 2, 2020
Merged

bpo-39776: Lock ++interp->tstate_next_unique_id. (GH-18746) #18746

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bpo-39776: Lock ++interp->tstate_next_unique_id.
skrah Mar 2, 2020
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skrah Mar 2, 2020
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6 changes: 6 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2020-03-02-20-12-33.bpo-39776.fNaxi_.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
Fix race condition where threads created by PyGILState_Ensure() could get a
duplicate id.

This affects consumers of tstate->id like the contextvar caching machinery,
which could return invalid cached objects under heavy thread load (observed
in embedded scenarios).
3 changes: 1 addition & 2 deletions Python/pystate.c
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Expand Up @@ -606,13 +606,12 @@ new_threadstate(PyInterpreterState *interp, int init)
tstate->context = NULL;
tstate->context_ver = 1;

tstate->id = ++interp->tstate_next_unique_id;

if (init) {
_PyThreadState_Init(tstate);
}

HEAD_LOCK(runtime);
tstate->id = ++interp->tstate_next_unique_id;
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@pitrou pitrou Mar 2, 2020

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Ha, is the reason for the problem that the increment was done unlocked?

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@skrah skrah Mar 2, 2020 •
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The location is unfortunate, but _PyThreadState_Init(tstate) does not use this field.

tstate->prev = NULL;
tstate->next = interp->tstate_head;
if (tstate->next)
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