python · ericsnowcurrently · Dec 16, 2023 · Dec 7, 2023 · Dec 7, 2023 · Dec 14, 2023
diff --git a/Include/internal/pycore_lock.h b/Include/internal/pycore_lock.h
@@ -213,6 +213,45 @@ _PyOnceFlag_CallOnce(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg)
     return _PyOnceFlag_CallOnceSlow(flag, fn, arg);
 }
 
+// A readers-writer (RW) lock. The lock supports multiple concurrent readers or
+// a single writer. The lock is write-preferring: if a writer is waiting while
+// the lock is read-locked then, new readers will be blocked. This avoids
+// starvation of writers.
+//
+// In C++, the equivalent synchronization primitive is std::shared_mutex
+// with shared ("read") and exclusive ("write") locking.
+//
+// The two least significant bits are used to indicate if the lock is
+// write-locked and if there are parked threads (either readers or writers)
+// waiting to acquire the lock. The remaining bits are used to indicate the
+// number of readers holding the lock.
+//
+// 0b000..00000: unlocked
+// 0bnnn..nnn00: nnn..nnn readers holding the lock
+// 0bnnn..nnn10: nnn..nnn readers holding the lock and a writer is waiting
+// 0b00000..010: unlocked with awoken writer about to acquire lock
+// 0b00000..001: write-locked
+// 0b00000..011: write-locked and readers or other writers are waiting
+//
+// Note that reader_count must be zero if the lock is held by a writer, and
+// vice versa. The lock can only be held by readers or a writer, but not both.
+//
+// The design is optimized for simplicity of the implementation. The lock is
+// not fair: if fairness is desired, use an additional PyMutex to serialize
+// writers. The lock is also not reentrant.
+typedef struct {
+    uintptr_t bits;
+} _PyRWMutex;
+
+// Read lock (i.e., shared lock)
+PyAPI_FUNC(void) _PyRWMutex_RLock(_PyRWMutex *rwmutex);
+PyAPI_FUNC(void) _PyRWMutex_RUnlock(_PyRWMutex *rwmutex);
+
+// Write lock (i.e., exclusive lock)
+PyAPI_FUNC(void) _PyRWMutex_Lock(_PyRWMutex *rwmutex);
+PyAPI_FUNC(void) _PyRWMutex_Unlock(_PyRWMutex *rwmutex);
+
+
 #ifdef __cplusplus
 }
 #endif

diff --git a/Modules/_testinternalcapi/test_lock.c b/Modules/_testinternalcapi/test_lock.c
@@ -372,6 +372,104 @@ test_lock_once(PyObject *self, PyObject *obj)
     Py_RETURN_NONE;
 }
 
+struct test_rwlock_data {
+    Py_ssize_t nthreads;
+    _PyRWMutex rw;
+    PyEvent step1;
+    PyEvent step2;
+    PyEvent step3;
+    PyEvent done;
+};
+
+static void
+rdlock_thread(void *arg)
+{
+    struct test_rwlock_data *test_data = arg;
+
+    // Acquire the lock in read mode
+    _PyRWMutex_RLock(&test_data->rw);
+    PyEvent_Wait(&test_data->step1);
+    _PyRWMutex_RUnlock(&test_data->rw);
+
+    _PyRWMutex_RLock(&test_data->rw);
+    PyEvent_Wait(&test_data->step3);
+    _PyRWMutex_RUnlock(&test_data->rw);
+
+    if (_Py_atomic_add_ssize(&test_data->nthreads, -1) == 1) {
+        _PyEvent_Notify(&test_data->done);
+    }
+}
+static void
+wrlock_thread(void *arg)
+{
+    struct test_rwlock_data *test_data = arg;
+
+    // First acquire the lock in write mode
+    _PyRWMutex_Lock(&test_data->rw);
+    PyEvent_Wait(&test_data->step2);
+    _PyRWMutex_Unlock(&test_data->rw);
+
+    if (_Py_atomic_add_ssize(&test_data->nthreads, -1) == 1) {
+        _PyEvent_Notify(&test_data->done);
+    }
+}
+
+static void
+wait_until(uintptr_t *ptr, uintptr_t value)
+{
+    // wait up to two seconds for *ptr == value
+    int iters = 0;
+    uintptr_t bits;
+    do {
+        pysleep(10);
+        bits = _Py_atomic_load_uintptr(ptr);
+        iters++;
+    } while (bits != value && iters < 200);
+}
+
+static PyObject *
+test_lock_rwlock(PyObject *self, PyObject *obj)
+{
+    struct test_rwlock_data test_data = {.nthreads = 3};
+
+    _PyRWMutex_Lock(&test_data.rw);
+    assert(test_data.rw.bits == 1);
+
+    _PyRWMutex_Unlock(&test_data.rw);
+    assert(test_data.rw.bits == 0);
+
+    // Start two readers
+    PyThread_start_new_thread(rdlock_thread, &test_data);
+    PyThread_start_new_thread(rdlock_thread, &test_data);
+
+    // wait up to two seconds for the threads to attempt to read-lock "rw"
+    wait_until(&test_data.rw.bits, 8);
+    assert(test_data.rw.bits == 8);
+
+    // start writer (while readers hold lock)
+    PyThread_start_new_thread(wrlock_thread, &test_data);
+    wait_until(&test_data.rw.bits, 10);
+    assert(test_data.rw.bits == 10);
+
+    // readers release lock, writer should acquire it
+    _PyEvent_Notify(&test_data.step1);
+    wait_until(&test_data.rw.bits, 3);
+    assert(test_data.rw.bits == 3);
+
+    // writer releases lock, readers acquire it
+    _PyEvent_Notify(&test_data.step2);
+    wait_until(&test_data.rw.bits, 8);
+    assert(test_data.rw.bits == 8);
+
+    // readers release lock again
+    _PyEvent_Notify(&test_data.step3);
+    wait_until(&test_data.rw.bits, 0);
+    assert(test_data.rw.bits == 0);
+
+    PyEvent_Wait(&test_data.done);
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef test_methods[] = {
     {"test_lock_basic", test_lock_basic, METH_NOARGS},
     {"test_lock_two_threads", test_lock_two_threads, METH_NOARGS},
@@ -380,6 +478,7 @@ static PyMethodDef test_methods[] = {
     _TESTINTERNALCAPI_BENCHMARK_LOCKS_METHODDEF
     {"test_lock_benchmark", test_lock_benchmark, METH_NOARGS},
     {"test_lock_once", test_lock_once, METH_NOARGS},
+    {"test_lock_rwlock", test_lock_rwlock, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 

diff --git a/Python/lock.c b/Python/lock.c
@@ -353,3 +353,109 @@ _PyOnceFlag_CallOnceSlow(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg)
         v = _Py_atomic_load_uint8(&flag->v);
     }
 }
+
+#define _Py_WRITE_LOCKED 1
+#define _PyRWMutex_READER_SHIFT 2
+#define _Py_RWMUTEX_MAX_READERS (UINTPTR_MAX >> _PyRWMutex_READER_SHIFT)
+
+static uintptr_t
+rwmutex_set_parked_and_wait(_PyRWMutex *rwmutex, uintptr_t bits)
+{
+    // Set _Py_HAS_PARKED and wait until we are woken up.
+    if ((bits & _Py_HAS_PARKED) == 0) {
+        uintptr_t newval = bits | _Py_HAS_PARKED;
+        if (!_Py_atomic_compare_exchange_uintptr(&rwmutex->bits,
+                                                 &bits, newval)) {
+            return bits;
+        }
+        bits = newval;
+    }
+
+    _PyParkingLot_Park(&rwmutex->bits, &bits, sizeof(bits), -1, NULL, 1);
+    return _Py_atomic_load_uintptr_relaxed(&rwmutex->bits);
+}
+
+// The number of readers holding the lock
+static uintptr_t
+rwmutex_reader_count(uintptr_t bits)
+{
+    return bits >> _PyRWMutex_READER_SHIFT;
+}
+
+void
+_PyRWMutex_RLock(_PyRWMutex *rwmutex)
+{
+    uintptr_t bits = _Py_atomic_load_uintptr_relaxed(&rwmutex->bits);
+    for (;;) {
+        if ((bits & _Py_WRITE_LOCKED)) {
+            // A writer already holds the lock.
+            bits = rwmutex_set_parked_and_wait(rwmutex, bits);
+            continue;
+        }
+        else if ((bits & _Py_HAS_PARKED)) {
+            // Reader(s) hold the lock (or just gave up the lock), but there is
+            // at least one waiting writer. We can't grab the lock because we
+            // don't want to starve the writer. Instead, we park ourselves and
+            // wait for the writer to eventually wake us up.
+            bits = rwmutex_set_parked_and_wait(rwmutex, bits);
+            continue;
+        }
+        else {
+            // The lock is unlocked or read-locked. Try to grab it.
+            assert(rwmutex_reader_count(bits) < _Py_RWMUTEX_MAX_READERS);
+            uintptr_t newval = bits + (1 << _PyRWMutex_READER_SHIFT);
+            if (!_Py_atomic_compare_exchange_uintptr(&rwmutex->bits,
+                                                     &bits, newval)) {
+                continue;
+            }
+            return;
+        }
+    }
+}
+
+void
+_PyRWMutex_RUnlock(_PyRWMutex *rwmutex)
+{
+    uintptr_t bits = _Py_atomic_add_uintptr(&rwmutex->bits, -(1 << _PyRWMutex_READER_SHIFT));
+    assert(rwmutex_reader_count(bits) > 0 && "lock was not read-locked");
+    bits -= (1 << _PyRWMutex_READER_SHIFT);
+
+    if (rwmutex_reader_count(bits) == 0 && (bits & _Py_HAS_PARKED)) {
+        _PyParkingLot_UnparkAll(&rwmutex->bits);
+        return;
+    }
+}
+
+void
+_PyRWMutex_Lock(_PyRWMutex *rwmutex)
+{
+    uintptr_t bits = _Py_atomic_load_uintptr_relaxed(&rwmutex->bits);
+    for (;;) {
+        // If there are no active readers and it's not already write-locked,
+        // then we can grab the lock.
+        if ((bits & ~_Py_HAS_PARKED) == 0) {
+            if (!_Py_atomic_compare_exchange_uintptr(&rwmutex->bits,
+                                                     &bits,
+                                                     bits | _Py_WRITE_LOCKED)) {
+                continue;
+            }
+            return;
+        }
+
+        // Otherwise, we have to wait.
+        bits = rwmutex_set_parked_and_wait(rwmutex, bits);
+    }
+}
+
+void
+_PyRWMutex_Unlock(_PyRWMutex *rwmutex)
+{
+    uintptr_t old_bits = _Py_atomic_exchange_uintptr(&rwmutex->bits, 0);
+
+    assert((old_bits & _Py_WRITE_LOCKED) && "lock was not write-locked");
+    assert(rwmutex_reader_count(old_bits) == 0 && "lock was read-locked");
+
+    if ((old_bits & _Py_HAS_PARKED) != 0) {
+        _PyParkingLot_UnparkAll(&rwmutex->bits);
+    }
+}