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gh-104372: Drop the GIL around the vfork() call. (#104782)

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On Linux where the `subprocess` module can use the `vfork` syscall for
faster spawning, prevent the parent process from blocking other threads
by dropping the GIL while it waits for the vfork'ed child process `exec`
outcome.  This prevents spawning a binary from a slow filesystem from
blocking the rest of the application.

Fixes #104372.
This commit is contained in:
Gregory P. Smith 2023-05-25 13:14:09 -07:00 • committed by GitHub
parent 08888650aa
commit d08679212d
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3 changed files with 31 additions and 6 deletions
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19
Modules/_posixsubprocess.c
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@ -559,7 +559,7 @@ reset_signal_handlers(const sigset_t *child_sigmask)
* required by POSIX but not supported natively on Linux. Another reason to
* avoid this family of functions is that sharing an address space between
* processes running with different privileges is inherently insecure.
* See bpo-35823 for further discussion and references.
* See https://bugs.python.org/issue35823 for discussion and references.
*
* In some C libraries, setrlimit() has the same thread list/signalling
* behavior since resource limits were per-thread attributes before
@ -798,6 +798,7 @@ do_fork_exec(char *const exec_array[],
pid_t pid;
#ifdef VFORK_USABLE
PyThreadState *vfork_tstate_save;
if (child_sigmask) {
/* These are checked by our caller; verify them in debug builds. */
assert(uid == (uid_t)-1);
@ -805,7 +806,22 @@ do_fork_exec(char *const exec_array[],
assert(extra_group_size < 0);
assert(preexec_fn == Py_None);
/* Drop the GIL so that other threads can continue execution while this
* thread in the parent remains blocked per vfork-semantics on the
* child's exec syscall outcome. Exec does filesystem access which
* can take an arbitrarily long time. This addresses GH-104372.
*
* The vfork'ed child still runs in our address space. Per POSIX it
* must be limited to nothing but exec, but the Linux implementation
* is a little more usable. See the child_exec() comment - The child
* MUST NOT re-acquire the GIL.
*/
vfork_tstate_save = PyEval_SaveThread();
pid = vfork();
if (pid != 0) {
// Not in the child process, reacquire the GIL.
PyEval_RestoreThread(vfork_tstate_save);
}
if (pid == (pid_t)-1) {
/* If vfork() fails, fall back to using fork(). When it isn't
* allowed in a process by the kernel, vfork can return -1
@ -819,6 +835,7 @@ do_fork_exec(char *const exec_array[],
}
if (pid != 0) {
// Parent process.
return pid;
}