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-rw-r--r--uts/common/fs/zfs/zrlock.c194
1 files changed, 194 insertions, 0 deletions
diff --git a/uts/common/fs/zfs/zrlock.c b/uts/common/fs/zfs/zrlock.c
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+++ b/uts/common/fs/zfs/zrlock.c
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+/*
+ * 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 (c) 2010, Oracle and/or its affiliates. All rights reserved.
+ */
+
+/*
+ * A Zero Reference Lock (ZRL) is a reference count that can lock out new
+ * references only when the count is zero and only without waiting if the count
+ * is not already zero. It is similar to a read-write lock in that it allows
+ * multiple readers and only a single writer, but it does not allow a writer to
+ * block while waiting for readers to exit, and therefore the question of
+ * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
+ * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
+ * is perfectly safe for the same reader to acquire the same lock multiple
+ * times. The fact that a ZRL is reentrant for readers (through multiple calls
+ * to zrl_add()) makes it convenient for determining whether something is
+ * actively referenced without the fuss of flagging lock ownership across
+ * function calls.
+ */
+#include <sys/zrlock.h>
+
+/*
+ * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
+ * treated as zero references.
+ */
+#define ZRL_LOCKED ((uint32_t)-1)
+#define ZRL_DESTROYED -2
+
+void
+zrl_init(zrlock_t *zrl)
+{
+ mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
+ zrl->zr_refcount = 0;
+ cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
+#ifdef ZFS_DEBUG
+ zrl->zr_owner = NULL;
+ zrl->zr_caller = NULL;
+#endif
+}
+
+void
+zrl_destroy(zrlock_t *zrl)
+{
+ ASSERT(zrl->zr_refcount == 0);
+
+ mutex_destroy(&zrl->zr_mtx);
+ zrl->zr_refcount = ZRL_DESTROYED;
+ cv_destroy(&zrl->zr_cv);
+}
+
+void
+#ifdef ZFS_DEBUG
+zrl_add_debug(zrlock_t *zrl, const char *zc)
+#else
+zrl_add(zrlock_t *zrl)
+#endif
+{
+ uint32_t n = (uint32_t)zrl->zr_refcount;
+
+ while (n != ZRL_LOCKED) {
+ uint32_t cas = atomic_cas_32(
+ (uint32_t *)&zrl->zr_refcount, n, n + 1);
+ if (cas == n) {
+ ASSERT((int32_t)n >= 0);
+#ifdef ZFS_DEBUG
+ if (zrl->zr_owner == curthread) {
+ DTRACE_PROBE2(zrlock__reentry,
+ zrlock_t *, zrl, uint32_t, n);
+ }
+ zrl->zr_owner = curthread;
+ zrl->zr_caller = zc;
+#endif
+ return;
+ }
+ n = cas;
+ }
+
+ mutex_enter(&zrl->zr_mtx);
+ while (zrl->zr_refcount == ZRL_LOCKED) {
+ cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
+ }
+ ASSERT(zrl->zr_refcount >= 0);
+ zrl->zr_refcount++;
+#ifdef ZFS_DEBUG
+ zrl->zr_owner = curthread;
+ zrl->zr_caller = zc;
+#endif
+ mutex_exit(&zrl->zr_mtx);
+}
+
+void
+zrl_remove(zrlock_t *zrl)
+{
+ uint32_t n;
+
+ n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
+ ASSERT((int32_t)n >= 0);
+#ifdef ZFS_DEBUG
+ if (zrl->zr_owner == curthread) {
+ zrl->zr_owner = NULL;
+ zrl->zr_caller = NULL;
+ }
+#endif
+}
+
+int
+zrl_tryenter(zrlock_t *zrl)
+{
+ uint32_t n = (uint32_t)zrl->zr_refcount;
+
+ if (n == 0) {
+ uint32_t cas = atomic_cas_32(
+ (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
+ if (cas == 0) {
+#ifdef ZFS_DEBUG
+ ASSERT(zrl->zr_owner == NULL);
+ zrl->zr_owner = curthread;
+#endif
+ return (1);
+ }
+ }
+
+ ASSERT((int32_t)n > ZRL_DESTROYED);
+
+ return (0);
+}
+
+void
+zrl_exit(zrlock_t *zrl)
+{
+ ASSERT(zrl->zr_refcount == ZRL_LOCKED);
+
+ mutex_enter(&zrl->zr_mtx);
+#ifdef ZFS_DEBUG
+ ASSERT(zrl->zr_owner == curthread);
+ zrl->zr_owner = NULL;
+ membar_producer(); /* make sure the owner store happens first */
+#endif
+ zrl->zr_refcount = 0;
+ cv_broadcast(&zrl->zr_cv);
+ mutex_exit(&zrl->zr_mtx);
+}
+
+int
+zrl_refcount(zrlock_t *zrl)
+{
+ ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
+
+ int n = (int)zrl->zr_refcount;
+ return (n <= 0 ? 0 : n);
+}
+
+int
+zrl_is_zero(zrlock_t *zrl)
+{
+ ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
+
+ return (zrl->zr_refcount <= 0);
+}
+
+int
+zrl_is_locked(zrlock_t *zrl)
+{
+ ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
+
+ return (zrl->zr_refcount == ZRL_LOCKED);
+}
+
+#ifdef ZFS_DEBUG
+kthread_t *
+zrl_owner(zrlock_t *zrl)
+{
+ return (zrl->zr_owner);
+}
+#endif