aboutsummaryrefslogtreecommitdiff
path: root/linuxthreads/spinlock.c
diff options
context:
space:
mode:
Diffstat (limited to 'linuxthreads/spinlock.c')
-rw-r--r--linuxthreads/spinlock.c59
1 files changed, 59 insertions, 0 deletions
diff --git a/linuxthreads/spinlock.c b/linuxthreads/spinlock.c
new file mode 100644
index 0000000..170d9ae
--- /dev/null
+++ b/linuxthreads/spinlock.c
@@ -0,0 +1,59 @@
+/* Linuxthreads - a simple clone()-based implementation of Posix */
+/* threads for Linux. */
+/* Copyright (C) 1998 Xavier Leroy (Xavier.Leroy@inria.fr) */
+/* */
+/* This program is free software; you can redistribute it and/or */
+/* modify it under the terms of the GNU Library 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 */
+/* GNU Library General Public License for more details. */
+
+/* Spin locks */
+
+#include <sched.h>
+#include <time.h>
+#include "pthread.h"
+#include "internals.h"
+#include "spinlock.h"
+
+/* This function is called if the inlined test-and-set in acquire() failed */
+
+/* The retry strategy is as follows:
+ - We test and set the spinlock MAX_SPIN_COUNT times, calling
+ sched_yield() each time. This gives ample opportunity for other
+ threads with priority >= our priority to make progress and
+ release the spinlock.
+ - If a thread with priority < our priority owns the spinlock,
+ calling sched_yield() repeatedly is useless, since we're preventing
+ the owning thread from making progress and releasing the spinlock.
+ So, after MAX_SPIN_LOCK attemps, we suspend the calling thread
+ using nanosleep(). This again should give time to the owning thread
+ for releasing the spinlock.
+ Notice that the nanosleep() interval must not be too small,
+ since the kernel does busy-waiting for short intervals in a realtime
+ process (!). The smallest duration that guarantees thread
+ suspension is currently 2ms.
+ - When nanosleep() returns, we try again, doing MAX_SPIN_COUNT
+ sched_yield(), then sleeping again if needed. */
+
+void __pthread_acquire(int * spinlock)
+{
+ int cnt = 0;
+ struct timespec tm;
+
+ while (testandset(spinlock)) {
+ if (cnt < MAX_SPIN_COUNT) {
+ sched_yield();
+ cnt++;
+ } else {
+ tm.tv_sec = 0;
+ tm.tv_nsec = SPIN_SLEEP_DURATION;
+ nanosleep(&tm, NULL);
+ cnt = 0;
+ }
+ }
+}