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e75ff35af2
Extract cross-platform condition variable support into a separate file and provide user-mode non-recursive locks for Windows.
271 lines
8.9 KiB
C
271 lines
8.9 KiB
C
/*
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* Implementation of the Global Interpreter Lock (GIL).
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*/
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#include <stdlib.h>
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#include <errno.h>
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/* First some general settings */
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/* microseconds (the Python API uses seconds, though) */
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#define DEFAULT_INTERVAL 5000
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static unsigned long gil_interval = DEFAULT_INTERVAL;
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#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
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/* Enable if you want to force the switching of threads at least every `gil_interval` */
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#undef FORCE_SWITCHING
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#define FORCE_SWITCHING
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/*
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Notes about the implementation:
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- The GIL is just a boolean variable (gil_locked) whose access is protected
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by a mutex (gil_mutex), and whose changes are signalled by a condition
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variable (gil_cond). gil_mutex is taken for short periods of time,
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and therefore mostly uncontended.
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- In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
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able to release the GIL on demand by another thread. A volatile boolean
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variable (gil_drop_request) is used for that purpose, which is checked
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at every turn of the eval loop. That variable is set after a wait of
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`interval` microseconds on `gil_cond` has timed out.
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[Actually, another volatile boolean variable (eval_breaker) is used
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which ORs several conditions into one. Volatile booleans are
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sufficient as inter-thread signalling means since Python is run
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on cache-coherent architectures only.]
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- A thread wanting to take the GIL will first let pass a given amount of
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time (`interval` microseconds) before setting gil_drop_request. This
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encourages a defined switching period, but doesn't enforce it since
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opcodes can take an arbitrary time to execute.
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The `interval` value is available for the user to read and modify
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using the Python API `sys.{get,set}switchinterval()`.
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- When a thread releases the GIL and gil_drop_request is set, that thread
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ensures that another GIL-awaiting thread gets scheduled.
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It does so by waiting on a condition variable (switch_cond) until
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the value of gil_last_holder is changed to something else than its
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own thread state pointer, indicating that another thread was able to
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take the GIL.
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This is meant to prohibit the latency-adverse behaviour on multi-core
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machines where one thread would speculatively release the GIL, but still
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run and end up being the first to re-acquire it, making the "timeslices"
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much longer than expected.
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(Note: this mechanism is enabled with FORCE_SWITCHING above)
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*/
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#include "condvar.h"
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#ifndef Py_HAVE_CONDVAR
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#error You need either a POSIX-compatible or a Windows system!
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#endif
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#define MUTEX_T PyMUTEX_T
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#define MUTEX_INIT(mut) \
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if (PyMUTEX_INIT(&(mut))) { \
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Py_FatalError("PyMUTEX_INIT(" #mut ") failed"); };
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#define MUTEX_FINI(mut) \
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if (PyMUTEX_FINI(&(mut))) { \
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Py_FatalError("PyMUTEX_FINI(" #mut ") failed"); };
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#define MUTEX_LOCK(mut) \
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if (PyMUTEX_LOCK(&(mut))) { \
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Py_FatalError("PyMUTEX_LOCK(" #mut ") failed"); };
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#define MUTEX_UNLOCK(mut) \
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if (PyMUTEX_UNLOCK(&(mut))) { \
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Py_FatalError("PyMUTEX_UNLOCK(" #mut ") failed"); };
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#define COND_T PyCOND_T
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#define COND_INIT(cond) \
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if (PyCOND_INIT(&(cond))) { \
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Py_FatalError("PyCOND_INIT(" #cond ") failed"); };
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#define COND_FINI(cond) \
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if (PyCOND_FINI(&(cond))) { \
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Py_FatalError("PyCOND_FINI(" #cond ") failed"); };
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#define COND_SIGNAL(cond) \
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if (PyCOND_SIGNAL(&(cond))) { \
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Py_FatalError("PyCOND_SIGNAL(" #cond ") failed"); };
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#define COND_WAIT(cond, mut) \
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if (PyCOND_WAIT(&(cond), &(mut))) { \
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Py_FatalError("PyCOND_WAIT(" #cond ") failed"); };
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#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
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{ \
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int r = PyCOND_TIMEDWAIT(&(cond), &(mut), (microseconds)); \
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if (r < 0) \
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Py_FatalError("PyCOND_WAIT(" #cond ") failed"); \
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if (r) /* 1 == timeout, 2 == impl. can't say, so assume timeout */ \
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timeout_result = 1; \
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else \
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timeout_result = 0; \
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} \
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/* Whether the GIL is already taken (-1 if uninitialized). This is atomic
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because it can be read without any lock taken in ceval.c. */
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static _Py_atomic_int gil_locked = {-1};
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/* Number of GIL switches since the beginning. */
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static unsigned long gil_switch_number = 0;
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/* Last PyThreadState holding / having held the GIL. This helps us know
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whether anyone else was scheduled after we dropped the GIL. */
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static _Py_atomic_address gil_last_holder = {NULL};
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/* This condition variable allows one or several threads to wait until
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the GIL is released. In addition, the mutex also protects the above
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variables. */
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static COND_T gil_cond;
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static MUTEX_T gil_mutex;
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#ifdef FORCE_SWITCHING
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/* This condition variable helps the GIL-releasing thread wait for
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a GIL-awaiting thread to be scheduled and take the GIL. */
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static COND_T switch_cond;
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static MUTEX_T switch_mutex;
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#endif
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static int gil_created(void)
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{
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return _Py_atomic_load_explicit(&gil_locked, _Py_memory_order_acquire) >= 0;
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}
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static void create_gil(void)
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{
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MUTEX_INIT(gil_mutex);
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#ifdef FORCE_SWITCHING
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MUTEX_INIT(switch_mutex);
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#endif
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COND_INIT(gil_cond);
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#ifdef FORCE_SWITCHING
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COND_INIT(switch_cond);
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#endif
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_Py_atomic_store_relaxed(&gil_last_holder, NULL);
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_Py_ANNOTATE_RWLOCK_CREATE(&gil_locked);
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_Py_atomic_store_explicit(&gil_locked, 0, _Py_memory_order_release);
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}
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static void destroy_gil(void)
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{
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/* some pthread-like implementations tie the mutex to the cond
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* and must have the cond destroyed first.
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*/
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COND_FINI(gil_cond);
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MUTEX_FINI(gil_mutex);
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#ifdef FORCE_SWITCHING
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COND_FINI(switch_cond);
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MUTEX_FINI(switch_mutex);
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#endif
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_Py_atomic_store_explicit(&gil_locked, -1, _Py_memory_order_release);
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_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
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}
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static void recreate_gil(void)
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{
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_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
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/* XXX should we destroy the old OS resources here? */
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create_gil();
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}
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static void drop_gil(PyThreadState *tstate)
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{
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if (!_Py_atomic_load_relaxed(&gil_locked))
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Py_FatalError("drop_gil: GIL is not locked");
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/* tstate is allowed to be NULL (early interpreter init) */
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if (tstate != NULL) {
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/* Sub-interpreter support: threads might have been switched
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under our feet using PyThreadState_Swap(). Fix the GIL last
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holder variable so that our heuristics work. */
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_Py_atomic_store_relaxed(&gil_last_holder, tstate);
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}
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MUTEX_LOCK(gil_mutex);
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_Py_ANNOTATE_RWLOCK_RELEASED(&gil_locked, /*is_write=*/1);
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_Py_atomic_store_relaxed(&gil_locked, 0);
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COND_SIGNAL(gil_cond);
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MUTEX_UNLOCK(gil_mutex);
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#ifdef FORCE_SWITCHING
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if (_Py_atomic_load_relaxed(&gil_drop_request) && tstate != NULL) {
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MUTEX_LOCK(switch_mutex);
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/* Not switched yet => wait */
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if (_Py_atomic_load_relaxed(&gil_last_holder) == tstate) {
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RESET_GIL_DROP_REQUEST();
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/* NOTE: if COND_WAIT does not atomically start waiting when
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releasing the mutex, another thread can run through, take
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the GIL and drop it again, and reset the condition
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before we even had a chance to wait for it. */
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COND_WAIT(switch_cond, switch_mutex);
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}
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MUTEX_UNLOCK(switch_mutex);
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}
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#endif
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}
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static void take_gil(PyThreadState *tstate)
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{
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int err;
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if (tstate == NULL)
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Py_FatalError("take_gil: NULL tstate");
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err = errno;
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MUTEX_LOCK(gil_mutex);
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if (!_Py_atomic_load_relaxed(&gil_locked))
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goto _ready;
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while (_Py_atomic_load_relaxed(&gil_locked)) {
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int timed_out = 0;
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unsigned long saved_switchnum;
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saved_switchnum = gil_switch_number;
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COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
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/* If we timed out and no switch occurred in the meantime, it is time
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to ask the GIL-holding thread to drop it. */
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if (timed_out &&
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_Py_atomic_load_relaxed(&gil_locked) &&
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gil_switch_number == saved_switchnum) {
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SET_GIL_DROP_REQUEST();
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}
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}
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_ready:
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#ifdef FORCE_SWITCHING
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/* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
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MUTEX_LOCK(switch_mutex);
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#endif
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/* We now hold the GIL */
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_Py_atomic_store_relaxed(&gil_locked, 1);
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_Py_ANNOTATE_RWLOCK_ACQUIRED(&gil_locked, /*is_write=*/1);
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if (tstate != _Py_atomic_load_relaxed(&gil_last_holder)) {
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_Py_atomic_store_relaxed(&gil_last_holder, tstate);
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++gil_switch_number;
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}
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#ifdef FORCE_SWITCHING
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COND_SIGNAL(switch_cond);
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MUTEX_UNLOCK(switch_mutex);
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#endif
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if (_Py_atomic_load_relaxed(&gil_drop_request)) {
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RESET_GIL_DROP_REQUEST();
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}
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if (tstate->async_exc != NULL) {
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_PyEval_SignalAsyncExc();
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}
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MUTEX_UNLOCK(gil_mutex);
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errno = err;
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}
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void _PyEval_SetSwitchInterval(unsigned long microseconds)
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{
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gil_interval = microseconds;
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}
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unsigned long _PyEval_GetSwitchInterval()
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{
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return gil_interval;
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}
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