unix: add spinlock.h

Add an atomic, non-blocking spinlock type.  The primary use case is to
have a mutex type that we can use while inside a signal handler.

Makefile.am

@@ -92,6 +92,7 @@ libuv_la_SOURCES += src/unix/async.c \
                    src/unix/poll.c \
                    src/unix/process.c \
                    src/unix/signal.c \
+                   src/unix/spinlock.h \
                    src/unix/stream.c \
                    src/unix/tcp.c \
                    src/unix/thread.c \

src/unix/spinlock.h

@@ -0,0 +1,53 @@
+/* Copyright (c) 2013, Ben Noordhuis <info@bnoordhuis.nl>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef UV_SPINLOCK_H_
+#define UV_SPINLOCK_H_
+
+#include "internal.h"  /* ACCESS_ONCE, UV_UNUSED */
+#include "atomic-ops.h"
+
+#define UV_SPINLOCK_INITIALIZER { 0 }
+
+typedef struct {
+  int lock;
+} uv_spinlock_t;
+
+UV_UNUSED(static void uv_spinlock_init(uv_spinlock_t* spinlock));
+UV_UNUSED(static void uv_spinlock_lock(uv_spinlock_t* spinlock));
+UV_UNUSED(static void uv_spinlock_unlock(uv_spinlock_t* spinlock));
+UV_UNUSED(static int uv_spinlock_trylock(uv_spinlock_t* spinlock));
+
+UV_UNUSED(static void uv_spinlock_init(uv_spinlock_t* spinlock)) {
+  ACCESS_ONCE(int, spinlock->lock) = 0;
+}
+
+UV_UNUSED(static void uv_spinlock_lock(uv_spinlock_t* spinlock)) {
+  while (!uv_spinlock_trylock(spinlock)) cpu_relax();
+}
+
+UV_UNUSED(static void uv_spinlock_unlock(uv_spinlock_t* spinlock)) {
+  ACCESS_ONCE(int, spinlock->lock) = 0;
+}
+
+UV_UNUSED(static int uv_spinlock_trylock(uv_spinlock_t* spinlock)) {
+  /* TODO(bnoordhuis) Maybe change to a ticket lock to guarantee fair queueing.
+   * Not really critical until we have locks that are (frequently) contended
+   * for by several threads.
+   */
+  return 0 == cmpxchgi(&spinlock->lock, 0, 1);
+}
+
+#endif  /* UV_SPINLOCK_H_ */

uv.gyp

@@ -146,6 +146,7 @@
             'src/unix/poll.c',
             'src/unix/process.c',
             'src/unix/signal.c',
+            'src/unix/spinlock.h',
             'src/unix/stream.c',
             'src/unix/tcp.c',
             'src/unix/thread.c',