Merge in the new GIL.

1 files changed, 335 insertions, 0 deletions

diff --git a/Python/ceval_gil.h b/Python/ceval_gil.h
new file mode 100644
index 00000000000..2687f9524da
--- /dev/null
+++ b/Python/ceval_gil.h
@@ -0,0 +1,335 @@
+/*
+ * Implementation of the Global Interpreter Lock (GIL).
+ */
+
+#include <stdlib.h>
+#include <errno.h>
+
+
+/* First some general settings */
+
+/* microseconds (the Python API uses seconds, though) */
+#define DEFAULT_INTERVAL 5000
+static unsigned long gil_interval = DEFAULT_INTERVAL;
+#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
+
+/* Enable if you want to force the switching of threads at least every `gil_interval` */
+#undef FORCE_SWITCHING
+#define FORCE_SWITCHING
+
+
+/*
+   Notes about the implementation:
+
+   - The GIL is just a boolean variable (gil_locked) whose access is protected
+     by a mutex (gil_mutex), and whose changes are signalled by a condition
+     variable (gil_cond). gil_mutex is taken for short periods of time,
+     and therefore mostly uncontended.
+
+   - In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
+     able to release the GIL on demand by another thread. A volatile boolean
+     variable (gil_drop_request) is used for that purpose, which is checked
+     at every turn of the eval loop. That variable is set after a wait of
+     `interval` microseconds on `gil_cond` has timed out.
+      
+      [Actually, another volatile boolean variable (eval_breaker) is used
+       which ORs several conditions into one. Volatile booleans are
+       sufficient as inter-thread signalling means since Python is run
+       on cache-coherent architectures only.]
+
+   - A thread wanting to take the GIL will first let pass a given amount of
+     time (`interval` microseconds) before setting gil_drop_request. This
+     encourages a defined switching period, but doesn't enforce it since
+     opcodes can take an arbitrary time to execute.
+ 
+     The `interval` value is available for the user to read and modify
+     using the Python API `sys.{get,set}switchinterval()`.
+
+   - When a thread releases the GIL and gil_drop_request is set, that thread
+     ensures that another GIL-awaiting thread gets scheduled.
+     It does so by waiting on a condition variable (switch_cond) until
+     the value of gil_last_holder is changed to something else than its
+     own thread state pointer, indicating that another thread was able to
+     take the GIL.
+ 
+     This is meant to prohibit the latency-adverse behaviour on multi-core
+     machines where one thread would speculatively release the GIL, but still
+     run and end up being the first to re-acquire it, making the "timeslices"
+     much longer than expected.
+     (Note: this mechanism is enabled with FORCE_SWITCHING above)
+*/
+
+#ifndef _POSIX_THREADS
+/* This means pthreads are not implemented in libc headers, hence the macro
+   not present in unistd.h. But they still can be implemented as an external
+   library (e.g. gnu pth in pthread emulation) */
+# ifdef HAVE_PTHREAD_H
+#  include <pthread.h> /* _POSIX_THREADS */
+# endif
+#endif
+
+
+#ifdef _POSIX_THREADS
+
+/*
+ * POSIX support
+ */
+
+#include <pthread.h>
+
+#define ADD_MICROSECONDS(tv, interval) \
+do { \
+    tv.tv_usec += (long) interval; \
+    tv.tv_sec += tv.tv_usec / 1000000; \
+    tv.tv_usec %= 1000000; \
+} while (0)
+
+/* We assume all modern POSIX systems have gettimeofday() */
+#ifdef GETTIMEOFDAY_NO_TZ
+#define GETTIMEOFDAY(ptv) gettimeofday(ptv)
+#else
+#define GETTIMEOFDAY(ptv) gettimeofday(ptv, (struct timezone *)NULL)
+#endif
+
+#define MUTEX_T pthread_mutex_t
+#define MUTEX_INIT(mut) \
+    if (pthread_mutex_init(&mut, NULL)) { \
+        Py_FatalError("pthread_mutex_init(" #mut ") failed"); };
+#define MUTEX_LOCK(mut) \
+    if (pthread_mutex_lock(&mut)) { \
+        Py_FatalError("pthread_mutex_lock(" #mut ") failed"); };
+#define MUTEX_UNLOCK(mut) \
+    if (pthread_mutex_unlock(&mut)) { \
+        Py_FatalError("pthread_mutex_unlock(" #mut ") failed"); };
+
+#define COND_T pthread_cond_t
+#define COND_INIT(cond) \
+    if (pthread_cond_init(&cond, NULL)) { \
+        Py_FatalError("pthread_cond_init(" #cond ") failed"); };
+#define COND_PREPARE(cond)
+#define COND_SIGNAL(cond) \
+    if (pthread_cond_signal(&cond)) { \
+        Py_FatalError("pthread_cond_signal(" #cond ") failed"); };
+#define COND_WAIT(cond, mut) \
+    if (pthread_cond_wait(&cond, &mut)) { \
+        Py_FatalError("pthread_cond_wait(" #cond ") failed"); };
+#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
+    { \
+        int r; \
+        struct timespec ts; \
+        struct timeval deadline; \
+        \
+        GETTIMEOFDAY(&deadline); \
+        ADD_MICROSECONDS(deadline, microseconds); \
+        ts.tv_sec = deadline.tv_sec; \
+        ts.tv_nsec = deadline.tv_usec * 1000; \
+        \
+        r = pthread_cond_timedwait(&cond, &mut, &ts); \
+        if (r == ETIMEDOUT) \
+            timeout_result = 1; \
+        else if (r) \
+            Py_FatalError("pthread_cond_timedwait(" #cond ") failed"); \
+        else \
+            timeout_result = 0; \
+    } \
+
+#elif defined(NT_THREADS)
+
+/*
+ * Windows (2000 and later, as well as (hopefully) CE) support
+ */
+
+#include <windows.h>
+
+#define MUTEX_T HANDLE
+#define MUTEX_INIT(mut) \
+    if (!(mut = CreateMutex(NULL, FALSE, NULL))) { \
+        Py_FatalError("CreateMutex(" #mut ") failed"); };
+#define MUTEX_LOCK(mut) \
+    if (WaitForSingleObject(mut, INFINITE) != WAIT_OBJECT_0) { \
+        Py_FatalError("WaitForSingleObject(" #mut ") failed"); };
+#define MUTEX_UNLOCK(mut) \
+    if (!ReleaseMutex(mut)) { \
+        Py_FatalError("ReleaseMutex(" #mut ") failed"); };
+
+/* We emulate condition variables with events. It is sufficient here.
+   (WaitForMultipleObjects() allows the event to be caught and the mutex
+   to be taken atomically) */
+#define COND_T HANDLE
+#define COND_INIT(cond) \
+    /* auto-reset, non-signalled */ \
+    if (!(cond = CreateEvent(NULL, FALSE, FALSE, NULL))) { \
+        Py_FatalError("CreateMutex(" #cond ") failed"); };
+#define COND_PREPARE(cond) \
+    if (!ResetEvent(cond)) { \
+        Py_FatalError("ResetEvent(" #cond ") failed"); };
+#define COND_SIGNAL(cond) \
+    if (!SetEvent(cond)) { \
+        Py_FatalError("SetEvent(" #cond ") failed"); };
+#define COND_WAIT(cond, mut) \
+    { \
+        DWORD r; \
+        HANDLE objects[2] = { cond, mut }; \
+        MUTEX_UNLOCK(mut); \
+        r = WaitForMultipleObjects(2, objects, TRUE, INFINITE); \
+        if (r != WAIT_OBJECT_0) \
+            Py_FatalError("WaitForSingleObject(" #cond ") failed"); \
+    }
+#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
+    { \
+        DWORD r; \
+        HANDLE objects[2] = { cond, mut }; \
+        MUTEX_UNLOCK(mut); \
+        r = WaitForMultipleObjects(2, objects, TRUE, microseconds / 1000); \
+        if (r == WAIT_TIMEOUT) { \
+            MUTEX_LOCK(mut); \
+            timeout_result = 1; \
+        } \
+        else if (r != WAIT_OBJECT_0) \
+            Py_FatalError("WaitForSingleObject(" #cond ") failed"); \
+        else \
+            timeout_result = 0; \
+    }
+
+#else
+
+#error You need either a POSIX-compatible or a Windows system!
+
+#endif /* _POSIX_THREADS, NT_THREADS */
+
+
+/* Whether the GIL is already taken (-1 if uninitialized). This is volatile
+   because it can be read without any lock taken in ceval.c. */
+static volatile int gil_locked = -1;
+/* Number of GIL switches since the beginning. */
+static unsigned long gil_switch_number = 0;
+/* Last thread holding / having held the GIL. This helps us know whether
+   anyone else was scheduled after we dropped the GIL. */
+static PyThreadState *gil_last_holder = NULL;
+
+/* This condition variable allows one or several threads to wait until
+   the GIL is released. In addition, the mutex also protects the above
+   variables. */
+static COND_T gil_cond;
+static MUTEX_T gil_mutex;
+
+#ifdef FORCE_SWITCHING
+/* This condition variable helps the GIL-releasing thread wait for
+   a GIL-awaiting thread to be scheduled and take the GIL. */
+static COND_T switch_cond;
+static MUTEX_T switch_mutex;
+#endif
+
+
+static int gil_created(void)
+{
+    return gil_locked >= 0;
+}
+
+static void create_gil(void)
+{
+    MUTEX_INIT(gil_mutex);
+#ifdef FORCE_SWITCHING
+    MUTEX_INIT(switch_mutex);
+#endif
+    COND_INIT(gil_cond);
+#ifdef FORCE_SWITCHING
+    COND_INIT(switch_cond);
+#endif
+    gil_locked = 0;
+    gil_last_holder = NULL;
+}
+
+static void recreate_gil(void)
+{
+    create_gil();
+}
+
+static void drop_gil(PyThreadState *tstate)
+{
+    /* NOTE: tstate is allowed to be NULL. */
+    if (!gil_locked)
+        Py_FatalError("drop_gil: GIL is not locked");
+    if (tstate != NULL && tstate != gil_last_holder)
+        Py_FatalError("drop_gil: wrong thread state");
+
+    MUTEX_LOCK(gil_mutex);
+    gil_locked = 0;
+    COND_SIGNAL(gil_cond);
+#ifdef FORCE_SWITCHING
+    COND_PREPARE(switch_cond);
+#endif
+    MUTEX_UNLOCK(gil_mutex);
+    
+#ifdef FORCE_SWITCHING
+    if (gil_drop_request) {
+        MUTEX_LOCK(switch_mutex);
+        /* Not switched yet => wait */
+        if (gil_last_holder == tstate)
+            COND_WAIT(switch_cond, switch_mutex);
+        MUTEX_UNLOCK(switch_mutex);
+    }
+#endif
+}
+
+static void take_gil(PyThreadState *tstate)
+{
+    int err;
+    if (tstate == NULL)
+        Py_FatalError("take_gil: NULL tstate");
+
+    err = errno;
+    MUTEX_LOCK(gil_mutex);
+
+    if (!gil_locked)
+        goto _ready;
+    
+    COND_PREPARE(gil_cond);
+    while (gil_locked) {
+        int timed_out = 0;
+        unsigned long saved_switchnum;
+
+        saved_switchnum = gil_switch_number;
+        COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
+        /* If we timed out and no switch occurred in the meantime, it is time
+           to ask the GIL-holding thread to drop it. */
+        if (timed_out && gil_locked && gil_switch_number == saved_switchnum) {
+            SET_GIL_DROP_REQUEST();
+        }
+    }
+_ready:
+#ifdef FORCE_SWITCHING
+    /* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
+    MUTEX_LOCK(switch_mutex);
+#endif
+    /* We now hold the GIL */
+    gil_locked = 1;
+
+    if (tstate != gil_last_holder) {
+        gil_last_holder = tstate;
+        ++gil_switch_number;
+    }
+#ifdef FORCE_SWITCHING
+    COND_SIGNAL(switch_cond);
+    MUTEX_UNLOCK(switch_mutex);
+#endif
+    if (gil_drop_request) {
+        RESET_GIL_DROP_REQUEST();
+    }
+    if (tstate->async_exc != NULL) {
+        _PyEval_SignalAsyncExc();
+    }
+    
+    MUTEX_UNLOCK(gil_mutex);
+    errno = err;
+}
+
+void _PyEval_SetSwitchInterval(unsigned long microseconds)
+{
+    gil_interval = microseconds;
+}
+
+unsigned long _PyEval_GetSwitchInterval()
+{
+    return gil_interval;
+}