1 files changed, 249 insertions, 0 deletions
diff --git a/module/zfs/rrwlock.c b/module/zfs/rrwlock.c
new file mode 100644
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--- /dev/null
+++ b/module/zfs/rrwlock.c
@@ -0,0 +1,249 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
+ * Use is subject to license terms.
+ */
+
+#pragma ident	"%Z%%M%	%I%	%E% SMI"
+
+#include <sys/refcount.h>
+#include <sys/rrwlock.h>
+
+/*
+ * This file contains the implementation of a re-entrant read
+ * reader/writer lock (aka "rrwlock").
+ *
+ * This is a normal reader/writer lock with the additional feature
+ * of allowing threads who have already obtained a read lock to
+ * re-enter another read lock (re-entrant read) - even if there are
+ * waiting writers.
+ *
+ * Callers who have not obtained a read lock give waiting writers priority.
+ *
+ * The rrwlock_t lock does not allow re-entrant writers, nor does it
+ * allow a re-entrant mix of reads and writes (that is, it does not
+ * allow a caller who has already obtained a read lock to be able to
+ * then grab a write lock without first dropping all read locks, and
+ * vice versa).
+ *
+ * The rrwlock_t uses tsd (thread specific data) to keep a list of
+ * nodes (rrw_node_t), where each node keeps track of which specific
+ * lock (rrw_node_t::rn_rrl) the thread has grabbed.  Since re-entering
+ * should be rare, a thread that grabs multiple reads on the same rrwlock_t
+ * will store multiple rrw_node_ts of the same 'rrn_rrl'. Nodes on the
+ * tsd list can represent a different rrwlock_t.  This allows a thread
+ * to enter multiple and unique rrwlock_ts for read locks at the same time.
+ *
+ * Since using tsd exposes some overhead, the rrwlock_t only needs to
+ * keep tsd data when writers are waiting.  If no writers are waiting, then
+ * a reader just bumps the anonymous read count (rr_anon_rcount) - no tsd
+ * is needed.  Once a writer attempts to grab the lock, readers then
+ * keep tsd data and bump the linked readers count (rr_linked_rcount).
+ *
+ * If there are waiting writers and there are anonymous readers, then a
+ * reader doesn't know if it is a re-entrant lock. But since it may be one,
+ * we allow the read to proceed (otherwise it could deadlock).  Since once
+ * waiting writers are active, readers no longer bump the anonymous count,
+ * the anonymous readers will eventually flush themselves out.  At this point,
+ * readers will be able to tell if they are a re-entrant lock (have a
+ * rrw_node_t entry for the lock) or not. If they are a re-entrant lock, then
+ * we must let the proceed.  If they are not, then the reader blocks for the
+ * waiting writers.  Hence, we do not starve writers.
+ */
+
+/* global key for TSD */
+uint_t rrw_tsd_key;
+
+typedef struct rrw_node {
+	struct rrw_node	*rn_next;
+	rrwlock_t	*rn_rrl;
+} rrw_node_t;
+
+static rrw_node_t *
+rrn_find(rrwlock_t *rrl)
+{
+	rrw_node_t *rn;
+
+	if (refcount_count(&rrl->rr_linked_rcount) == 0)
+		return (NULL);
+
+	for (rn = tsd_get(rrw_tsd_key); rn != NULL; rn = rn->rn_next) {
+		if (rn->rn_rrl == rrl)
+			return (rn);
+	}
+	return (NULL);
+}
+
+/*
+ * Add a node to the head of the singly linked list.
+ */
+static void
+rrn_add(rrwlock_t *rrl)
+{
+	rrw_node_t *rn;
+
+	rn = kmem_alloc(sizeof (*rn), KM_SLEEP);
+	rn->rn_rrl = rrl;
+	rn->rn_next = tsd_get(rrw_tsd_key);
+	VERIFY(tsd_set(rrw_tsd_key, rn) == 0);
+}
+
+/*
+ * If a node is found for 'rrl', then remove the node from this
+ * thread's list and return TRUE; otherwise return FALSE.
+ */
+static boolean_t
+rrn_find_and_remove(rrwlock_t *rrl)
+{
+	rrw_node_t *rn;
+	rrw_node_t *prev = NULL;
+
+	if (refcount_count(&rrl->rr_linked_rcount) == 0)
+		return (NULL);
+
+	for (rn = tsd_get(rrw_tsd_key); rn != NULL; rn = rn->rn_next) {
+		if (rn->rn_rrl == rrl) {
+			if (prev)
+				prev->rn_next = rn->rn_next;
+			else
+				VERIFY(tsd_set(rrw_tsd_key, rn->rn_next) == 0);
+			kmem_free(rn, sizeof (*rn));
+			return (B_TRUE);
+		}
+		prev = rn;
+	}
+	return (B_FALSE);
+}
+
+void
+rrw_init(rrwlock_t *rrl)
+{
+	mutex_init(&rrl->rr_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&rrl->rr_cv, NULL, CV_DEFAULT, NULL);
+	rrl->rr_writer = NULL;
+	refcount_create(&rrl->rr_anon_rcount);
+	refcount_create(&rrl->rr_linked_rcount);
+	rrl->rr_writer_wanted = B_FALSE;
+}
+
+void
+rrw_destroy(rrwlock_t *rrl)
+{
+	mutex_destroy(&rrl->rr_lock);
+	cv_destroy(&rrl->rr_cv);
+	ASSERT(rrl->rr_writer == NULL);
+	refcount_destroy(&rrl->rr_anon_rcount);
+	refcount_destroy(&rrl->rr_linked_rcount);
+}
+
+static void
+rrw_enter_read(rrwlock_t *rrl, void *tag)
+{
+	mutex_enter(&rrl->rr_lock);
+	ASSERT(rrl->rr_writer != curthread);
+	ASSERT(refcount_count(&rrl->rr_anon_rcount) >= 0);
+
+	while (rrl->rr_writer || (rrl->rr_writer_wanted &&
+	    refcount_is_zero(&rrl->rr_anon_rcount) &&
+	    rrn_find(rrl) == NULL))
+		cv_wait(&rrl->rr_cv, &rrl->rr_lock);
+
+	if (rrl->rr_writer_wanted) {
+		/* may or may not be a re-entrant enter */
+		rrn_add(rrl);
+		(void) refcount_add(&rrl->rr_linked_rcount, tag);
+	} else {
+		(void) refcount_add(&rrl->rr_anon_rcount, tag);
+	}
+	ASSERT(rrl->rr_writer == NULL);
+	mutex_exit(&rrl->rr_lock);
+}
+
+static void
+rrw_enter_write(rrwlock_t *rrl)
+{
+	mutex_enter(&rrl->rr_lock);
+	ASSERT(rrl->rr_writer != curthread);
+
+	while (refcount_count(&rrl->rr_anon_rcount) > 0 ||
+	    refcount_count(&rrl->rr_linked_rcount) > 0 ||
+	    rrl->rr_writer != NULL) {
+		rrl->rr_writer_wanted = B_TRUE;
+		cv_wait(&rrl->rr_cv, &rrl->rr_lock);
+	}
+	rrl->rr_writer_wanted = B_FALSE;
+	rrl->rr_writer = curthread;
+	mutex_exit(&rrl->rr_lock);
+}
+
+void
+rrw_enter(rrwlock_t *rrl, krw_t rw, void *tag)
+{
+	if (rw == RW_READER)
+		rrw_enter_read(rrl, tag);
+	else
+		rrw_enter_write(rrl);
+}
+
+void
+rrw_exit(rrwlock_t *rrl, void *tag)
+{
+	mutex_enter(&rrl->rr_lock);
+	ASSERT(!refcount_is_zero(&rrl->rr_anon_rcount) ||
+	    !refcount_is_zero(&rrl->rr_linked_rcount) ||
+	    rrl->rr_writer != NULL);
+
+	if (rrl->rr_writer == NULL) {
+		if (rrn_find_and_remove(rrl)) {
+			if (refcount_remove(&rrl->rr_linked_rcount, tag) == 0)
+				cv_broadcast(&rrl->rr_cv);
+
+		} else {
+			if (refcount_remove(&rrl->rr_anon_rcount, tag) == 0)
+				cv_broadcast(&rrl->rr_cv);
+		}
+	} else {
+		ASSERT(rrl->rr_writer == curthread);
+		ASSERT(refcount_is_zero(&rrl->rr_anon_rcount) &&
+		    refcount_is_zero(&rrl->rr_linked_rcount));
+		rrl->rr_writer = NULL;
+		cv_broadcast(&rrl->rr_cv);
+	}
+	mutex_exit(&rrl->rr_lock);
+}
+
+boolean_t
+rrw_held(rrwlock_t *rrl, krw_t rw)
+{
+	boolean_t held;
+
+	mutex_enter(&rrl->rr_lock);
+	if (rw == RW_WRITER) {
+		held = (rrl->rr_writer == curthread);
+	} else {
+		held = (!refcount_is_zero(&rrl->rr_anon_rcount) ||
+		    !refcount_is_zero(&rrl->rr_linked_rcount));
+	}
+	mutex_exit(&rrl->rr_lock);
+
+	return (held);
+}