* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
+ * The init_once implementation is adapted from the Pthreads-win32 library
+ * implementation based on MCS (Mellor-Crummy Scott) locks, originally
+ * distributed with the following copyright and permission notice:
+ *
+ * Pthreads-win32 - POSIX Threads Library for Win32
+ * Copyright(C) 1998 John E. Bossom
+ * Copyright(C) 1999,2005 Pthreads-win32 contributors
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
-#include "windows-once.h"
static DWORD tls_key;
#define tls_key_valid (tls_key != TLS_OUT_OF_INDEXES)
return TlsSetValue(tls_key, p);
}
-static void init_once_with_tls(void)
+/*
+ * Synchronize with another thread's call to init_once_wait on the same
+ * HANDLE object, which must be initialized to null. If init_once_wait
+ * got there first, we receive a handle to the event object which can
+ * then be signaled.
+ */
+static void init_once_signal(HANDLE *ep)
{
- tls_key = TlsAlloc();
- init_once_cb();
+ /*
+ * Note that INVALID_HANDLE_VALUE is distinct from a null pointer
+ * and also distinct from any handle returned by CreateEvent.
+ */
+ HANDLE e = INVALID_HANDLE_VALUE;
+
+ if ((e = InterlockedCompareExchangePointer(ep, e, NULL)))
+ SetEvent(e);
}
+/*
+ * Synchronize with another thread's call to init_once_signal on the same
+ * HANDLE object.
+ */
+static void init_once_wait(HANDLE *ep)
+{
+#if _WIN64
+ if (!InterlockedAdd64((LONG64 *)ep, 0)) // load with memory barrier
+#else
+ if (!InterlockedAdd((LONG *)ep, 0))
+#endif
+ {
+ HANDLE e;
+
+ e = CreateEvent(NULL, FALSE, FALSE, NULL);
+ if (!InterlockedCompareExchangePointer(ep, e, NULL))
+ WaitForSingleObject(e, INFINITE);
+ CloseHandle(e);
+ }
+}
+
+/*
+ * This implementation using an MCS lock variation requires only a single
+ * pointer of shared global state initialized to null, and in the uncontended
+ * case does not require allocation of any Windows resources whatsoever.
+ *
+ * These locks are described in the paper:
+ *
+ * "Algorithms for Scalable Synchronization on Shared-Memory Multiprocessors"
+ * by John M. Mellor-Crummey and Michael L. Scott.
+ * ACM Transactions on Computer Systems Volume 9, Issue 1 (Feb. 1991).
+ *
+ * The basic idea is that each thread has a local state, which includes a
+ * pointer to the next waiting thread. The global state is a tail pointer to
+ * the last waiting thread. The running thread holds the lock and also the
+ * pointer to the first waiter.
+ *
+ * On acquire, atomically swap our fresh local state with the global tail
+ * pointer, becoming the new last waiter. We receive a pointer to the previous
+ * last waiter (or nothing, in the unlocked case). At this point it is safe
+ * for a new thread to come along and update the tail pointer again. If
+ * needed, we then update the last waiter to point to our thread, signal
+ * that this is completed, and then wait to be signaled.
+ *
+ * On release, if the tail pointer points to us there are no waiters, and this
+ * can be confirmed with an atomic compare and exchange to the done state,
+ * which is equivalent to the original state except that a subsequent acquirer
+ * will know that the initialization has been previously completed.
+ *
+ * If that didn't unlock the lock, we need to wait for the signal from the next
+ * thread (which may not have updated our next pointer yet), then signal the
+ * next thread to wake up. Eventually the queue will empty and the lock is
+ * left in the done state, at which point a simple atomic load can determine
+ * that nothing else needs to happen.
+ */
static int init_once(void)
{
- static glwthread_once_t ctrl = GLWTHREAD_ONCE_INIT;
+ struct once_state * const init_done = (void *)-1;
+
+ struct once_state {
+ struct once_state *next;
+ HANDLE ready_event, next_event;
+ };
+
+ static void * volatile tail;
+ struct once_state local = {0};
+ struct once_state *p;
+
+ /* fast path for the normal (init completed) case. */
+ if (tail == init_done)
+ return 1;
+
+ if (!(p = InterlockedExchangePointer(&tail, &local))) {
+ /* we're number one! */
+ tls_key = TlsAlloc();
+ init_once_cb();
+ } else if (p != init_done) {
+ /* contended, wait for predecessor */
+ p->next = &local;
+ init_once_signal(&p->next_event);
+ init_once_wait(&local.ready_event);
+ }
+
+ p = InterlockedCompareExchangePointer(&tail, init_done, &local);
+ if (p != &local) {
+ /* contended, wait for successor */
+ init_once_wait(&local.next_event);
+ init_once_signal(&local.next->ready_event);
+ }
- glwthread_once(&ctrl, init_once_with_tls);
return 1;
}
projects / cdecl99.git / blobdiff