Implement shared/exclusive locks.

Reviewed by:	bmilekic, jake, jhb

1 files changed, 160 insertions, 0 deletions

diff --git a/sys/kern/kern_sx.c b/sys/kern/kern_sx.c
new file mode 100644
index 0000000..582ff04
--- /dev/null
+++ b/sys/kern/kern_sx.c
@@ -0,0 +1,160 @@
+/*
+ * Copyright (C) 2001 Jason Evans <jasone@freebsd.org>.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice(s), this list of conditions and the following disclaimer as
+ *    the first lines of this file unmodified other than the possible 
+ *    addition of one or more copyright notices.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice(s), this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+ * DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * Shared/exclusive locks.  This implementation assures deterministic lock
+ * granting behavior, so that slocks and xlocks are interleaved.
+ *
+ * Priority propagation will not generally raise the priority of lock holders,
+ * so should not be relied upon in combination with sx locks.
+ *
+ * The witness code can not detect lock cycles.
+ *
+ *   slock --> xlock (deadlock)
+ *   slock --> slock (slock recursion, not fatal)
+ *   xlock --> xlock (deadlock)
+ *   xlock --> slock (deadlock)
+ */
+
+#include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/ktr.h>
+#include <sys/condvar.h>
+#include <sys/mutex.h>
+#include <sys/sx.h>
+
+void
+sx_init(struct sx *sx, const char *description)
+{
+
+	mtx_init(&sx->sx_lock, description, MTX_DEF);
+	sx->sx_cnt = 0;
+	cv_init(&sx->sx_shrd_cv, description);
+	sx->sx_shrd_wcnt = 0;
+	cv_init(&sx->sx_excl_cv, description);
+	sx->sx_excl_wcnt = 0;
+}
+
+void
+sx_destroy(struct sx *sx)
+{
+
+	KASSERT((sx->sx_cnt == 0 && sx->sx_shrd_wcnt == 0 && sx->sx_excl_wcnt ==
+	    0), ("%s: holders or waiters\n", __FUNCTION__));
+
+	mtx_destroy(&sx->sx_lock);
+	cv_destroy(&sx->sx_shrd_cv);
+	cv_destroy(&sx->sx_excl_cv);
+}
+
+void
+sx_slock(struct sx *sx)
+{
+
+	mtx_lock(&sx->sx_lock);
+
+	/*
+	 * Loop in case we lose the race for lock acquisition.
+	 */
+	while (sx->sx_cnt < 0) {
+		sx->sx_shrd_wcnt++;
+		cv_wait(&sx->sx_shrd_cv, &sx->sx_lock);
+		sx->sx_shrd_wcnt--;
+	}
+
+	/* Acquire a shared lock. */
+	sx->sx_cnt++;
+
+	mtx_unlock(&sx->sx_lock);
+}
+
+void
+sx_xlock(struct sx *sx)
+{
+
+	mtx_lock(&sx->sx_lock);
+
+	/* Loop in case we lose the race for lock acquisition. */
+	while (sx->sx_cnt != 0) {
+		sx->sx_excl_wcnt++;
+		cv_wait(&sx->sx_excl_cv, &sx->sx_lock);
+		sx->sx_excl_wcnt--;
+	}
+
+	/* Acquire an exclusive lock. */
+	sx->sx_cnt--;
+
+	mtx_unlock(&sx->sx_lock);
+}
+
+void
+sx_sunlock(struct sx *sx)
+{
+
+	mtx_lock(&sx->sx_lock);
+	KASSERT((sx->sx_cnt > 0), ("%s: lacking slock\n", __FUNCTION__));
+
+	/* Release. */
+	sx->sx_cnt--;
+
+	/*
+	 * If we just released the last shared lock, wake any waiters up, giving
+	 * exclusive lockers precedence.  In order to make sure that exclusive
+	 * lockers won't be blocked forever, don't wake shared lock waiters if
+	 * there are exclusive lock waiters.
+	 */
+	if (sx->sx_excl_wcnt > 0) {
+		if (sx->sx_cnt == 0)
+			cv_signal(&sx->sx_excl_cv);
+	} else if (sx->sx_shrd_wcnt > 0)
+		cv_broadcast(&sx->sx_shrd_cv);
+
+	mtx_unlock(&sx->sx_lock);
+}
+
+void
+sx_xunlock(struct sx *sx)
+{
+
+	mtx_lock(&sx->sx_lock);
+	KASSERT((sx->sx_cnt == -1), ("%s: lacking xlock\n", __FUNCTION__));
+
+	/* Release. */
+	sx->sx_cnt++;
+
+	/*
+	 * Wake up waiters if there are any.  Give precedence to slock waiters.
+	 */
+	if (sx->sx_shrd_wcnt > 0)
+		cv_broadcast(&sx->sx_shrd_cv);
+	else if (sx->sx_excl_wcnt > 0)
+		cv_signal(&sx->sx_excl_cv);
+
+	mtx_unlock(&sx->sx_lock);
+}