CRITICAL PATCH!

We occasionally received reports from people getting "invalid tstate"
crashes (this is a fatal error in PyThreadState_Delete()).  Finally
several people were able to reproduce it reliably and Tim Peters
discovered that there is a race condition when multiple threads are
calling this function without holding the global interpreter lock (the
function may be called without holding that).

Solved the race condition by adding a lock around the mutating uses of
interp->tstate_head.  Tim and Jonathan Giddy have run tests that make
it likely that this fixes the crashes -- although Tim hasn't heard
from the person who reported the original problem.

diff --git a/Python/pystate.c b/Python/pystate.c
index 338c0c31dfcace0e7c1efb68ef3e78e6417ef6e1..b62fe7cd20506469056946594b9fe1196b02f42a 100644 (file)
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -40,6 +40,18 @@ PERFORMANCE OF THIS SOFTWARE.
 }
 
 
+#ifdef WITH_THREAD
+#include "pythread.h"
+static PyThread_type_lock head_mutex = NULL; /* Protects interp->tstate_head */
+#define HEAD_INIT() (head_mutex || (head_mutex = PyThread_allocate_lock()))
+#define HEAD_LOCK() PyThread_acquire_lock(head_mutex, WAIT_LOCK)
+#define HEAD_UNLOCK() PyThread_release_lock(head_mutex)
+#else
+#define HEAD_INIT() /* Nothing */
+#define HEAD_LOCK() /* Nothing */
+#define HEAD_UNLOCK() /* Nothing */
+#endif
+
 static PyInterpreterState *interp_head = NULL;
 
 PyThreadState *_PyThreadState_Current = NULL;
@@ -51,6 +63,7 @@ PyInterpreterState_New()
        PyInterpreterState *interp = PyMem_NEW(PyInterpreterState, 1);
 
        if (interp != NULL) {
+               HEAD_INIT();
                interp->modules = NULL;
                interp->sysdict = NULL;
                interp->builtins = NULL;
@@ -70,8 +83,10 @@ PyInterpreterState_Clear(interp)
        PyInterpreterState *interp;
 {
        PyThreadState *p;
+       HEAD_LOCK();
        for (p = interp->tstate_head; p != NULL; p = p->next)
                PyThreadState_Clear(p);
+       HEAD_UNLOCK();
        ZAP(interp->modules);
        ZAP(interp->sysdict);
        ZAP(interp->builtins);
@@ -82,12 +97,11 @@ static void
 zapthreads(interp)
        PyInterpreterState *interp;
 {
-       PyThreadState *p, *q;
-       p = interp->tstate_head;
-       while (p != NULL) {
-               q = p->next;
+       PyThreadState *p;
+       /* No need to lock the mutex here because this should only happen
+          when the threads are all really dead (XXX famous last words). */
+       while ((p = interp->tstate_head) != NULL) {
                PyThreadState_Delete(p);
-               p = q;
        }
 }
 
@@ -139,8 +153,10 @@ PyThreadState_New(interp)
                tstate->sys_profilefunc = NULL;
                tstate->sys_tracefunc = NULL;
 
+               HEAD_LOCK();
                tstate->next = interp->tstate_head;
                interp->tstate_head = tstate;
+               HEAD_UNLOCK();
        }
 
        return tstate;
@@ -185,6 +201,7 @@ PyThreadState_Delete(tstate)
        interp = tstate->interp;
        if (interp == NULL)
                Py_FatalError("PyThreadState_Delete: NULL interp");
+       HEAD_LOCK();
        for (p = &interp->tstate_head; ; p = &(*p)->next) {
                if (*p == NULL)
                        Py_FatalError(
@@ -193,6 +210,7 @@ PyThreadState_Delete(tstate)
                        break;
        }
        *p = tstate->next;
+       HEAD_UNLOCK();
        PyMem_DEL(tstate);
 }