python
diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
@@ -2094,6 +2094,24 @@ Socket Objects
 
       For further information, please consult the :ref:`notes on socket timeouts <socket-timeouts>`.
 
+      .. note::
+
+         Do not call :meth:`setblocking` or :meth:`settimeout` while another
+         thread is performing an operation on the same socket.  Changing the
+         blocking mode concurrently with operations such as :meth:`connect`,
+         :meth:`accept`, :meth:`recv`, or :meth:`send` can cause the operation
+         to block unexpectedly or to fail with a non-blocking error.
+
+         Applications must use external synchronization when changing a
+         socket's blocking mode or timeout.  This also applies to socket
+         objects and file descriptors that share the same underlying socket,
+         since the blocking mode is shared at the operating-system level.
+
+         Reading a socket's current timeout or blocking mode with
+         :meth:`gettimeout` or :meth:`getblocking` remains safe to do
+         concurrently, only *changing* it during an operation requires the
+         synchronization described above.
+
       .. versionchanged:: 3.7
          The method no longer toggles :const:`SOCK_NONBLOCK` flag on
          :attr:`socket.type`.

diff --git a/Include/internal/pycore_pyatomic_ft_wrappers.h b/Include/internal/pycore_pyatomic_ft_wrappers.h
@@ -53,6 +53,10 @@ extern "C" {
     _Py_atomic_load_uint64_acquire(&value)
 #define FT_ATOMIC_LOAD_UINT64_RELAXED(value) \
     _Py_atomic_load_uint64_relaxed(&value)
+#define FT_ATOMIC_LOAD_INT64_ACQUIRE(value) \
+    ((int64_t)_Py_atomic_load_uint64_acquire((const uint64_t *)&value))
+#define FT_ATOMIC_STORE_INT64_RELEASE(value, new_value) \
+    _Py_atomic_store_uint64_release((uint64_t *)&value, (uint64_t)new_value)
 #define FT_ATOMIC_LOAD_ULONG_RELAXED(value) \
     _Py_atomic_load_ulong_relaxed(&value)
 #define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) \
@@ -159,6 +163,8 @@ extern "C" {
 #define FT_ATOMIC_LOAD_UINT32_RELAXED(value) value
 #define FT_ATOMIC_LOAD_UINT64_ACQUIRE(value) value
 #define FT_ATOMIC_LOAD_UINT64_RELAXED(value) value
+#define FT_ATOMIC_LOAD_INT64_ACQUIRE(value) value
+#define FT_ATOMIC_STORE_INT64_RELEASE(value, new_value) value = new_value
 #define FT_ATOMIC_LOAD_ULONG_RELAXED(value) value
 #define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) value = new_value
 #define FT_ATOMIC_STORE_PTR_RELEASE(value, new_value) value = new_value

diff --git a/Lib/test/test_free_threading/test_socket.py b/Lib/test/test_free_threading/test_socket.py
@@ -0,0 +1,46 @@
+import socket
+import unittest
+
+from test import support
+from test.support import threading_helper
+
+N = 200 if support.check_sanitizer(thread=True) else 1000
+
+
+@threading_helper.requires_working_threading()
+class TestSocketTimeoutRaces(unittest.TestCase):
+    # gh-153852: the socket timeout was read/written non-atomically, so reading
+    # it with gettimeout()/getblocking() raced with settimeout()/setblocking()
+    # on the same socket.
+
+    def new_socket(self):
+        s = socket.socket()
+        self.addCleanup(s.close)
+        return s
+
+    def check_race(self, read, write):
+        def reader():
+            for _ in range(N):
+                read()
+
+        def writer():
+            for _ in range(N):
+                write()
+
+        threading_helper.run_concurrently([reader, writer])
+
+    def test_gettimeout_vs_setblocking(self):
+        s = self.new_socket()
+        self.check_race(s.gettimeout, lambda: s.setblocking(False))
+
+    def test_gettimeout_vs_settimeout(self):
+        s = self.new_socket()
+        self.check_race(s.gettimeout, lambda: s.settimeout(1.0))
+
+    def test_getblocking_vs_settimeout(self):
+        s = self.new_socket()
+        self.check_race(s.getblocking, lambda: s.settimeout(0))
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2026-07-18-15-30-56.gh-issue-153935.xpkjEc.rst b/Misc/NEWS.d/next/Library/2026-07-18-15-30-56.gh-issue-153935.xpkjEc.rst
@@ -0,0 +1 @@
+Fix a data race on a socket's timeout in the :term:`free-threaded build`.
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
@@ -110,6 +110,7 @@ Local naming conventions:
 #include "pycore_fileutils.h"     // _Py_set_inheritable()
 #include "pycore_moduleobject.h"  // _PyModule_GetState
 #include "pycore_object.h"        // _PyObject_VisitType()
+#include "pycore_pyatomic_ft_wrappers.h"  // FT_ATOMIC_LOAD_INT64_ACQUIRE()
 #include "pycore_time.h"          // _PyTime_AsMilliseconds()
 #include "pycore_tuple.h"         // _PyTuple_FromPairSteal
 #include "pycore_pystate.h"       // _Py_AssertHoldsTstate()
@@ -591,6 +592,12 @@ get_sock_fd(PySocketSockObject *s)
 #endif
 }
 
+static inline PyTime_t
+get_sock_timeout(PySocketSockObject *s)
+{
+    return FT_ATOMIC_LOAD_INT64_ACQUIRE(s->sock_timeout);
+}
+
 #define _PySocketSockObject_CAST(op)    ((PySocketSockObject *)(op))
 
 static inline socket_state *
@@ -681,7 +688,7 @@ class _socket.socket "PySocketSockObject *" "clinic_state()->sock_type"
 #else
 /* If there's no timeout left, we don't have to call select, so it's a safe,
  * little white lie. */
-#define IS_SELECTABLE(s) (_PyIsSelectable_fd((s)->sock_fd) || (s)->sock_timeout <= 0)
+#define IS_SELECTABLE(s) (_PyIsSelectable_fd((s)->sock_fd) || get_sock_timeout(s) <= 0)
 #endif
 
 // SCM_RIGHTS, sendmsg(), recvmsg() and sethostname() don't work properly on
@@ -1068,7 +1075,7 @@ sock_call_ex(PySocketSockObject *s,
             /* retry sock_func() */
         }
 
-        if (s->sock_timeout > 0
+        if (get_sock_timeout(s) > 0
             && (CHECK_ERRNO(EWOULDBLOCK) || CHECK_ERRNO(EAGAIN))) {
             /* False positive: sock_func() failed with EWOULDBLOCK or EAGAIN.
 
@@ -1094,7 +1101,8 @@ sock_call(PySocketSockObject *s,
           int (*func) (PySocketSockObject *s, void *data),
           void *data)
 {
-    return sock_call_ex(s, writing, func, data, 0, NULL, s->sock_timeout);
+    return sock_call_ex(s, writing, func, data, 0, NULL,
+                        get_sock_timeout(s));
 }
 
 
@@ -1158,6 +1166,9 @@ new_sockobject(socket_state *state, SOCKET_T fd, int family, int type,
     if (s == NULL) {
         return NULL;
     }
+#ifdef Py_GIL_DISABLED
+    s->sock_timeout_mutex = (PyMutex){0};
+#endif
     if (init_sockobject(state, s, fd, family, type, proto) == -1) {
         Py_DECREF(s);
         return NULL;
@@ -3168,9 +3179,15 @@ sock_setblocking(PyObject *self, PyObject *arg)
     if (block < 0)
         return NULL;
 
-   PySocketSockObject *s = _PySocketSockObject_CAST(self);
-    s->sock_timeout = _PyTime_FromSeconds(block ? -1 : 0);
-    if (internal_setblocking(s, block) == -1) {
+    PySocketSockObject *s = _PySocketSockObject_CAST(self);
+    FT_MUTEX_LOCK(&s->sock_timeout_mutex);
+    int result = internal_setblocking(s, block);
+    if (result == 0) {
+        FT_ATOMIC_STORE_INT64_RELEASE(
+            s->sock_timeout, _PyTime_FromSeconds(block ? -1 : 0));
+    }
+    FT_MUTEX_UNLOCK(&s->sock_timeout_mutex);
+    if (result < 0) {
         return NULL;
     }
     Py_RETURN_NONE;
@@ -3190,8 +3207,8 @@ setblocking(False) is equivalent to settimeout(0.0).");
 static PyObject *
 sock_getblocking(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
-   PySocketSockObject *s = _PySocketSockObject_CAST(self);
-    if (s->sock_timeout) {
+    PySocketSockObject *s = _PySocketSockObject_CAST(self);
+    if (get_sock_timeout(s)) {
         Py_RETURN_TRUE;
     }
     else {
@@ -3261,8 +3278,6 @@ sock_settimeout(PyObject *self, PyObject *arg)
         return NULL;
 
     PySocketSockObject *s = _PySocketSockObject_CAST(self);
-    s->sock_timeout = timeout;
-
     int block = timeout < 0;
     /* Blocking mode for a Python socket object means that operations
        like :meth:`recv` or :meth:`sendall` will block the execution of
@@ -3285,7 +3300,13 @@ sock_settimeout(PyObject *self, PyObject *arg)
         ``> 0``              ``True``              non-blocking
     */
 
-    if (internal_setblocking(s, block) == -1) {
+    FT_MUTEX_LOCK(&s->sock_timeout_mutex);
+    int result = internal_setblocking(s, block);
+    if (result == 0) {
+        FT_ATOMIC_STORE_INT64_RELEASE(s->sock_timeout, timeout);
+    }
+    FT_MUTEX_UNLOCK(&s->sock_timeout_mutex);
+    if (result < 0) {
         return NULL;
     }
     Py_RETURN_NONE;
@@ -3305,11 +3326,12 @@ static PyObject *
 sock_gettimeout_impl(PyObject *self, void *Py_UNUSED(ignored))
 {
     PySocketSockObject *s = _PySocketSockObject_CAST(self);
-    if (s->sock_timeout < 0) {
+    PyTime_t sock_timeout = get_sock_timeout(s);
+    if (sock_timeout < 0) {
         Py_RETURN_NONE;
     }
     else {
-        double seconds = PyTime_AsSecondsDouble(s->sock_timeout);
+        double seconds = PyTime_AsSecondsDouble(sock_timeout);
         return PyFloat_FromDouble(seconds);
     }
 }
@@ -3706,10 +3728,11 @@ internal_connect(PySocketSockObject *s, struct sockaddr *addr, int addrlen,
            If the socket is non-blocking, raise InterruptedError. The caller is
            responsible to wait until the connection completes, fails or timed
            out (it's the case in asyncio for example). */
-        wait_connect = (s->sock_timeout != 0 && IS_SELECTABLE(s));
+        wait_connect = (get_sock_timeout(s) != 0 && IS_SELECTABLE(s));
     }
     else {
-        wait_connect = (s->sock_timeout > 0 && err == SOCK_INPROGRESS_ERR
+        wait_connect = (get_sock_timeout(s) > 0
+                        && err == SOCK_INPROGRESS_ERR
                         && IS_SELECTABLE(s));
     }
 
@@ -3727,13 +3750,13 @@ internal_connect(PySocketSockObject *s, struct sockaddr *addr, int addrlen,
     if (raise) {
         /* socket.connect() raises an exception on error */
         if (sock_call_ex(s, 1, sock_connect_impl, NULL,
-                         1, NULL, s->sock_timeout) < 0)
+                         1, NULL, get_sock_timeout(s)) < 0)
             return -1;
     }
     else {
         /* socket.connect_ex() returns the error code on error */
         if (sock_call_ex(s, 1, sock_connect_impl, NULL,
-                         1, &err, s->sock_timeout) < 0)
+                         1, &err, get_sock_timeout(s)) < 0)
             return err;
     }
     return 0;
@@ -4688,8 +4711,8 @@ _socket_socket_sendall_impl(PySocketSockObject *s, Py_buffer *pbuf,
     char *buf;
     Py_ssize_t len, n;
     struct sock_send ctx;
-    int has_timeout = (s->sock_timeout > 0);
-    PyTime_t timeout = s->sock_timeout;
+    PyTime_t timeout = get_sock_timeout(s);
+    int has_timeout = (timeout > 0);
     PyTime_t deadline = 0;
     int deadline_initialized = 0;
     PyObject *res = NULL;
@@ -5604,6 +5627,9 @@ sock_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     if (new != NULL) {
         ((PySocketSockObject *)new)->sock_fd = INVALID_SOCKET;
         ((PySocketSockObject *)new)->sock_timeout = _PyTime_FromSeconds(-1);
+#ifdef Py_GIL_DISABLED
+        ((PySocketSockObject *)new)->sock_timeout_mutex = (PyMutex){0};
+#endif
         ((PySocketSockObject *)new)->errorhandler = &set_error;
 #ifdef MS_WINDOWS
         ((PySocketSockObject *)new)->quickack = 0;

diff --git a/Modules/socketmodule.h b/Modules/socketmodule.h
@@ -346,6 +346,9 @@ typedef struct {
                                         sets a Python exception */
     PyTime_t sock_timeout;     /* Operation timeout in seconds;
                                         0.0 means non-blocking */
+#ifdef Py_GIL_DISABLED
+    PyMutex sock_timeout_mutex;
+#endif
     struct _socket_state *state;
 #ifdef MS_WINDOWS
     int quickack;
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix a data race on a socket's timeout in the :term:`free-threaded build`.