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GeezerVison is showing github.com — gh-151707: fix race conditions in `FileIO` by KowalskiThomas · Pull Request #151708 · python/cpython

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@KowalskiThomas

@KowalskiThomas KowalskiThomas commented Jun 19, 2026 •
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What is this PR?

Fixes #151707.

Note that the PR itself does a bit more than just fix the race condition. It introduces new helper functions around the self->fd to avoid race conditions altogether, in the same fashion as the socketmodule.c file (and updates every relevant usage site to call into those helpers).

cpython/Modules/socketmodule.c

Lines 566 to 592 in 46b5e3e

static inline void
set_sock_fd(PySocketSockObject *s, SOCKET_T fd)
{
#if SIZEOF_SOCKET_T == SIZEOF_INT
_Py_atomic_store_int_relaxed((int *)&s->sock_fd, (int)fd);
#elif SIZEOF_SOCKET_T == SIZEOF_LONG
_Py_atomic_store_long_relaxed((long *)&s->sock_fd, (long)fd);
#elif SIZEOF_SOCKET_T == SIZEOF_LONG_LONG
_Py_atomic_store_llong_relaxed((long long *)&s->sock_fd, (long long)fd);
#else
#error "Unsupported SIZEOF_SOCKET_T"
#endif
}
static inline SOCKET_T
get_sock_fd(PySocketSockObject *s)
{
#if SIZEOF_SOCKET_T == SIZEOF_INT
return (SOCKET_T)_Py_atomic_load_int_relaxed((int *)&s->sock_fd);
#elif SIZEOF_SOCKET_T == SIZEOF_LONG
return (SOCKET_T)_Py_atomic_load_long_relaxed((long *)&s->sock_fd);
#elif SIZEOF_SOCKET_T == SIZEOF_LONG_LONG
return (SOCKET_T)_Py_atomic_load_llong_relaxed((long long *)&s->sock_fd);
#else
#error "Unsupported SIZEOF_SOCKET_T"
#endif
}

  • Issue: Race condition in FileIO under free-threading #151707

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KowalskiThomas added 2 commits June 19, 2026 10:55
@KowalskiThomas
fix: race conditions in FileIO
3a68a17
@KowalskiThomas
test: add regression test
14ed8f1
@bedevere-app bedevere-app Bot mentioned this pull request Jun 19, 2026
Race condition in FileIO under free-threading #151707
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vstinner
vstinner reviewed Jun 19, 2026
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I'm not sure that it's the right approach. While the change makes sure that int fd integer is not modified in parallel, it doesn't ensure that the file descriptor is not closed before/while we are using it.

For example, FileIO.isatty() gets int fd, check that it's >= 0, and then call isatty().

If another thread calls close(fd) in parallel, after >= 0 check and before isatty() call, isatty() fails with EBADF.

It's even worse if the file descriptor is recycled for a new unrelated file.

Maybe it would be safer to the whole method in a critical section. It would prevent calling FileIO.close() in parallel: the call would be serialized.

@cmaloney: What do you think?

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@vstinner

vstinner commented Jun 19, 2026

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Oh, the TSan tests failed. Example:

WARNING: ThreadSanitizer: data race (pid=20851)
  Write of size 8 at 0x72b000000060 by thread T9189:
    #0 close <null> (python+0x1044c7) (BuildId: ac94b2b86fdb18d0b904ed36fb1fd97b68fe5a64)
    #1 internal_close /home/runner/work/cpython/cpython/./Modules/_io/fileio.c:155:15 (python+0x731b66) (BuildId: ac94b2b86fdb18d0b904ed36fb1fd97b68fe5a64)
    #2 _io_FileIO_close_impl /home/runner/work/cpython/cpython/./Modules/_io/fileio.c:210:10 (python+0x730fbb) (BuildId: ac94b2b86fdb18d0b904ed36fb1fd97b68fe5a64)
    #3 _io_FileIO_close /home/runner/work/cpython/cpython/./Modules/_io/clinic/fileio.c.h:34:12 (python+0x730fbb)
...
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@KowalskiThomas

KowalskiThomas commented Jun 19, 2026

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I'm not sure that it's the right approach.

Oh, the TSan tests failed.

I guess we have an answer 😁
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@github-actions

github-actions Bot commented Sep 23, 2026

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This PR is stale because it has been open for 90 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label Sep 23, 2026
@scasella scasella mentioned this pull request Sep 26, 2026
Free-threading: concurrent FileIO close/truncate can double-free stat_atopen #158196
Open
@BHUVANSH855 BHUVANSH855 mentioned this pull request Oct 10, 2026
gh-158196: Fix double free of FileIO stat_atopen #159153
Open
@KowalskiThomas

KowalskiThomas commented Oct 21, 2026

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I'm going to close this PR as I'm working (in an issue I'll post soon) on a better way to make FileIO thread-safe.
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@KowalskiThomas KowalskiThomas closed this Oct 21, 2026
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@vstinner vstinner vstinner left review comments
@cmaloney cmaloney Awaiting requested review from cmaloney cmaloney will be requested when the pull request is marked ready for review cmaloney is a code owner

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