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Thread-safe one-time initialization #111956

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Tracked by #108219
colesbury opened this issue Nov 10, 2023 · 0 comments
Closed
Tracked by #108219

Thread-safe one-time initialization #111956

colesbury opened this issue Nov 10, 2023 · 0 comments
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@colesbury
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3.13 new features, bugs and security fixes topic-free-threading type-feature A feature request or enhancement

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@colesbury
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colesbury commented Nov 10, 2023 •
edited by bedevere-app bot

Feature or enhancement

Some CPython internals require initialization exactly once. Some of these one time initializations are not thread-safe without the GIL or have data races according to the C11 memory model.

We should add a lightweight, thread-safe one-time initialization API similar to C++11's std::call_once 1. The proposed internal-only API follows C++11's std::call_once, but adapted for C (i.e., error returns and function pointers):

typedef struct {
    uint8_t v;
} _PyOnceFlag;

typedef int _Py_once_fn_t(void *arg);

// Calls `fn` once using `flag`. The `arg` is passed to the call to `fn`.
//
// Returns 1 on success and 0 on failure.
//
// If `fn` returns 1 (success), then subsequent calls immediately return 1.
// If `fn` returns 0 (failure), then subsequent calls will retry the call.
int _PyOnceFlag_CallOnce(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg);

As an example, the Python-ast.c relies on the GIL and an initialized variable to ensure that it is only initialized once:

cpython/Python/Python-ast.c

Lines 1126 to 1135 in d61313b

static int
init_types(struct ast_state *state)
{
// init_types() must not be called after _PyAST_Fini()
// has been called
assert(state->initialized >= 0);
if (state->initialized) {
return 1;
}

Linked PRs

Footnotes

  1. Also, pthread_once and C11's call_once. std::call_once supports error returns, which is important for CPython's use cases. ↩
@colesbury colesbury added type-feature A feature request or enhancement 3.13 new features, bugs and security fixes topic-free-threading labels Nov 10, 2023
@colesbury colesbury mentioned this issue Nov 10, 2023
Open
colesbury added a commit to colesbury/cpython that referenced this issue Nov 10, 2023
@colesbury
pythongh-111956: Add thread-safe one-time initialization.
141222f 
The one-time initialization (`_PyOnceFlag`) is used in two places:

 * `Python/Python-ast.c`
 * `Python/getargs.c`
colesbury added a commit to colesbury/cpython that referenced this issue Nov 10, 2023
@colesbury
pythongh-111956: Add thread-safe one-time initialization.
c5dad28 
The one-time initialization (`_PyOnceFlag`) is used in two places:

 * `Python/Python-ast.c`
 * `Python/getargs.c`
@bedevere-app bedevere-app bot mentioned this issue Nov 10, 2023
Merged
@colesbury colesbury self-assigned this Nov 10, 2023
@colesbury colesbury mentioned this issue Nov 14, 2023
Open
ericsnowcurrently pushed a commit that referenced this issue Nov 16, 2023
@colesbury
gh-111956: Add thread-safe one-time initialization. (gh-111960)
446f18a
aisk pushed a commit to aisk/cpython that referenced this issue Feb 11, 2024
@colesbury @aisk
pythongh-111956: Add thread-safe one-time initialization. (pythongh-1…
a8a97ad 
…11960)
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