New submission from Paul Ganssle <p.gans...@gmail.com>:
I was recently debugging some multithreaded Rust code that was deadlocking, and
I tracked it down to what I'm fairly certain is a bug somewhere in
PyCapsule_Import, where it seems that PyCapsule_Import releases the GIL without
first acquiring it.
I've attached a MWE of a multi-threaded application that is able to
simultaneously acquire the GIL twice. The relevant portion is here:
void *acquire_gil(void *arg) {
bool import = ((arg_t *)arg)->import;
int n = ((arg_t *)arg)->id;
printf("Waiting for GIL (%d)\n", n);
int gstate = PyGILState_Ensure();
printf("Gil acquired! (%d)\n", n);
usleep(125000);
if (import) {
PyCapsule_Import(CAPSULE_NAME, 0);
}
usleep(125000);
PyGILState_Release(gstate);
printf("Gil released! (%d)\n", n);
return NULL;
}
If you run it with `./gil` such that the PyCapsule_Import call is never
reached, you get:
Waiting for GIL (0)
Gil acquired! (0)
Waiting for GIL (1)
Gil released! (0)
Gil acquired! (1)
Gil released! (1)
However, if you run with `./gil import` such that PyCapsule_Import is reached,
you get (emphasis mine):
Waiting for GIL (0)
Gil acquired! (0)
Waiting for GIL (1)
**Gil acquired! (1)**
**Gil released! (1)**
Gil released! (0)
For convenience sake, I have created a small repo with a make file for the PoC:
https://github.com/pganssle/capsule-gil-poc
I have tested this on version 3.6.6 and 3.7.0. The makefile works in a
virtualenv, but you have to manually tweak the PY_MAJOR and PY_MINOR variables
in Makefile because I didn't get too fancy with it.
----------
files: main.c
messages: 323620
nosy: p-ganssle, pablogsal
priority: normal
severity: normal
status: open
title: PyCapsule_Import seems to release the GIL without acquiring it
versions: Python 3.6, Python 3.7
Added file: https://bugs.python.org/file47753/main.c
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<https://bugs.python.org/issue34416>
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