diff --git a/Misc/NEWS.d/next/Library/2024-11-20-11-37-08.gh-issue-126316.ElkZmE.rst b/Misc/NEWS.d/next/Library/2024-11-20-11-37-08.gh-issue-126316.ElkZmE.rst
new file mode 100644
index 00000000000000..d643254c5b3564
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-11-20-11-37-08.gh-issue-126316.ElkZmE.rst
@@ -0,0 +1,2 @@
+:mod:`grp`: Make :func:`grp.getgrall` thread-safe by adding a mutex. Patch
+by Victor Stinner.
diff --git a/Modules/clinic/grpmodule.c.h b/Modules/clinic/grpmodule.c.h
index cc0ad210f42743..43700a11c15c91 100644
--- a/Modules/clinic/grpmodule.c.h
+++ b/Modules/clinic/grpmodule.c.h
@@ -2,35 +2,11 @@
 preserve
 [clinic start generated code]*/
 
-PyDoc_STRVAR(grp_getgrgid__doc__,
-"getgrgid($module, /, id)\n"
-"--\n"
-"\n"
-"Return the group database entry for the given numeric group ID.\n"
-"\n"
-"If id is not valid, raise KeyError.");
-
-#define GRP_GETGRGID_METHODDEF    \
-    {"getgrgid", (PyCFunction)(void(*)(void))grp_getgrgid, METH_VARARGS|METH_KEYWORDS, grp_getgrgid__doc__},
-
-static PyObject *
-grp_getgrgid_impl(PyObject *module, PyObject *id);
-
-static PyObject *
-grp_getgrgid(PyObject *module, PyObject *args, PyObject *kwargs)
-{
-    PyObject *return_value = NULL;
-    static char *_keywords[] = {"id", NULL};
-    PyObject *id;
-
-    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O:getgrgid", _keywords,
-        &id))
-        goto exit;
-    return_value = grp_getgrgid_impl(module, id);
-
-exit:
-    return return_value;
-}
+#if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
+#  include "pycore_gc.h"          // PyGC_Head
+#  include "pycore_runtime.h"     // _Py_ID()
+#endif
+#include "pycore_modsupport.h"    // _PyArg_UnpackKeywords()
 
 PyDoc_STRVAR(grp_getgrnam__doc__,
 "getgrnam($module, /, name)\n"
@@ -41,21 +17,52 @@ PyDoc_STRVAR(grp_getgrnam__doc__,
 "If name is not valid, raise KeyError.");
 
 #define GRP_GETGRNAM_METHODDEF    \
-    {"getgrnam", (PyCFunction)(void(*)(void))grp_getgrnam, METH_VARARGS|METH_KEYWORDS, grp_getgrnam__doc__},
+    {"getgrnam", _PyCFunction_CAST(grp_getgrnam), METH_FASTCALL|METH_KEYWORDS, grp_getgrnam__doc__},
 
 static PyObject *
 grp_getgrnam_impl(PyObject *module, PyObject *name);
 
 static PyObject *
-grp_getgrnam(PyObject *module, PyObject *args, PyObject *kwargs)
+grp_getgrnam(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
 {
     PyObject *return_value = NULL;
-    static char *_keywords[] = {"name", NULL};
+    #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
+
+    #define NUM_KEYWORDS 1
+    static struct {
+        PyGC_Head _this_is_not_used;
+        PyObject_VAR_HEAD
+        PyObject *ob_item[NUM_KEYWORDS];
+    } _kwtuple = {
+        .ob_base = PyVarObject_HEAD_INIT(&PyTuple_Type, NUM_KEYWORDS)
+        .ob_item = { &_Py_ID(name), },
+    };
+    #undef NUM_KEYWORDS
+    #define KWTUPLE (&_kwtuple.ob_base.ob_base)
+
+    #else  // !Py_BUILD_CORE
+    #  define KWTUPLE NULL
+    #endif  // !Py_BUILD_CORE
+
+    static const char * const _keywords[] = {"name", NULL};
+    static _PyArg_Parser _parser = {
+        .keywords = _keywords,
+        .fname = "getgrnam",
+        .kwtuple = KWTUPLE,
+    };
+    #undef KWTUPLE
+    PyObject *argsbuf[1];
     PyObject *name;
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "U:getgrnam", _keywords,
-        &name))
+    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser, 1, 1, 0, argsbuf);
+    if (!args) {
+        goto exit;
+    }
+    if (!PyUnicode_Check(args[0])) {
+        _PyArg_BadArgument("getgrnam", "argument 'name'", "str", args[0]);
         goto exit;
+    }
+    name = args[0];
     return_value = grp_getgrnam_impl(module, name);
 
 exit:
@@ -82,4 +89,4 @@ grp_getgrall(PyObject *module, PyObject *Py_UNUSED(ignored))
 {
     return grp_getgrall_impl(module);
 }
-/*[clinic end generated code: output=81f180beb67fc585 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=1168333f1b15de11 input=a9049054013a1b77]*/
diff --git a/Modules/grpmodule.c b/Modules/grpmodule.c
index f7d3e12f347ec2..e0534576ff1e82 100644
--- a/Modules/grpmodule.c
+++ b/Modules/grpmodule.c
@@ -1,9 +1,8 @@
 /* UNIX group file access module */
 
-// Need limited C API version 3.13 for PyMem_RawRealloc()
-#include "pyconfig.h"   // Py_GIL_DISABLED
-#ifndef Py_GIL_DISABLED
-#define Py_LIMITED_API 0x030d0000
+// Argument Clinic uses the internal C API
+#ifndef Py_BUILD_CORE_BUILTIN
+#  define Py_BUILD_CORE_MODULE 1
 #endif
 
 #include "Python.h"
@@ -110,20 +109,15 @@ mkgrent(PyObject *module, struct group *p)
     return v;
 }
 
-/*[clinic input]
-grp.getgrgid
-
-    id: object
-
-Return the group database entry for the given numeric group ID.
-
-If id is not valid, raise KeyError.
-[clinic start generated code]*/
-
 static PyObject *
-grp_getgrgid_impl(PyObject *module, PyObject *id)
-/*[clinic end generated code: output=30797c289504a1ba input=15fa0e2ccf5cda25]*/
+grp_getgrgid(PyObject *module, PyObject *args, PyObject *kwargs)
 {
+    static char *kwlist[] = {"id", NULL};
+    PyObject *id;
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O", kwlist, &id)) {
+        return NULL;
+    }
+
     PyObject *retval = NULL;
     int nomem = 0;
     char *buf = NULL, *buf2 = NULL;
@@ -190,6 +184,15 @@ grp_getgrgid_impl(PyObject *module, PyObject *id)
     return retval;
 }
 
+PyDoc_STRVAR(grp_getgrgid__doc__,
+"getgrgid($module, /, id)\n"
+"--\n"
+"\n"
+"Return the group database entry for the given numeric group ID.\n"
+"\n"
+"If id is not valid, raise KeyError.");
+
+
 /*[clinic input]
 grp.getgrnam
 
@@ -281,28 +284,38 @@ static PyObject *
 grp_getgrall_impl(PyObject *module)
 /*[clinic end generated code: output=585dad35e2e763d7 input=d7df76c825c367df]*/
 {
-    PyObject *d;
-    struct group *p;
-
-    if ((d = PyList_New(0)) == NULL)
+    PyObject *d = PyList_New(0);
+    if (d == NULL) {
         return NULL;
+    }
+
+    static PyMutex getgrall_mutex = {0};
+    PyMutex_Lock(&getgrall_mutex);
     setgrent();
+
+    struct group *p;
     while ((p = getgrent()) != NULL) {
+        // gh-126316: Don't release the mutex around mkgrent() since
+        // setgrent()/endgrent() are not reentrant / thread-safe. A deadlock
+        // is unlikely since mkgrent() should not be able to call arbitrary
+        // Python code.
         PyObject *v = mkgrent(module, p);
         if (v == NULL || PyList_Append(d, v) != 0) {
             Py_XDECREF(v);
-            Py_DECREF(d);
-            endgrent();
-            return NULL;
+            Py_CLEAR(d);
+            goto done;
         }
         Py_DECREF(v);
     }
+
+done:
     endgrent();
+    PyMutex_Unlock(&getgrall_mutex);
     return d;
 }
 
 static PyMethodDef grp_methods[] = {
-    GRP_GETGRGID_METHODDEF
+    {"getgrgid", _PyCFunction_CAST(grp_getgrgid), METH_VARARGS|METH_KEYWORDS, grp_getgrgid__doc__},
     GRP_GETGRNAM_METHODDEF
     GRP_GETGRALL_METHODDEF
     {NULL, NULL}
diff --git a/Tools/c-analyzer/cpython/ignored.tsv b/Tools/c-analyzer/cpython/ignored.tsv
index 260eaa4f14f767..5e271a5014cb35 100644
--- a/Tools/c-analyzer/cpython/ignored.tsv
+++ b/Tools/c-analyzer/cpython/ignored.tsv
@@ -742,6 +742,7 @@ Modules/expat/xmlrole.c	-	declClose	-
 Modules/expat/xmlrole.c	-	error	-
 
 ## other
+Modules/grpmodule.c	grp_getgrall_impl	getgrall_mutex	-
 Modules/_io/_iomodule.c	-	_PyIO_Module	-
 Modules/_sqlite/module.c	-	_sqlite3module	-
 Modules/clinic/md5module.c.h	_md5_md5	_keywords	-