diff --git a/kmod/src/Makefile b/kmod/src/Makefile
index e31924ed..83da3832 100644
--- a/kmod/src/Makefile
+++ b/kmod/src/Makefile
@@ -3,5 +3,5 @@ obj-$(CONFIG_SCOUTFS_FS) := scoutfs.o
 CFLAGS_scoutfs_trace.o = -I$(src) # define_trace.h double include
  
 scoutfs-y += alloc.o bio.o compact.o counters.o data.o dir.o kvec.o inode.o \
-	     ioctl.o item.o key.o manifest.o msg.o seg.o scoutfs_trace.o \
-	     super.o trans.o treap.o xattr.o
+	     ioctl.o item.o key.o lock.o manifest.o msg.o seg.o \
+	     scoutfs_trace.o super.o trans.o treap.o xattr.o
diff --git a/kmod/src/item.c b/kmod/src/item.c
index 83e1a09c..e489005f 100644
--- a/kmod/src/item.c
+++ b/kmod/src/item.c
@@ -24,6 +24,7 @@
 #include "seg.h"
 #include "counters.h"
 #include "scoutfs_trace.h"
+#include "trans.h"
 
 /*
  * A simple rbtree of cached items isolates the item API callers from
@@ -1536,6 +1537,79 @@ int scoutfs_item_dirty_seg(struct super_block *sb, struct scoutfs_segment *seg)
 	return 0;
 }
 
+/*
+ * The caller wants us to write out any dirty items within the given
+ * range.  We look for any dirty items within the range and if we find
+ * any we issue a sync which writes out all the dirty items.
+ */
+int scoutfs_item_writeback(struct super_block *sb,
+			   struct scoutfs_key_buf *start,
+			   struct scoutfs_key_buf *end)
+{
+	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
+	struct item_cache *cac = sbi->item_cache;
+	struct cached_item *item;
+	unsigned long flags;
+	bool sync = false;
+	int ret = 0;
+
+	/* XXX think about racing with trans write */
+
+	spin_lock_irqsave(&cac->lock, flags);
+
+	if (cac->nr_dirty_items) {
+		item = next_item(&cac->items, start);
+		if (item && !(item->dirty & ITEM_DIRTY))
+			item = next_dirty(item);
+		if (item && scoutfs_key_compare(item->key, end) <= 0)
+			sync = true;
+	}
+
+	spin_unlock_irqrestore(&cac->lock, flags);
+
+	if (sync)
+		ret = scoutfs_sync_fs(sb, 1);
+
+	return ret;
+}
+
+/*
+ * The caller wants us to drop any items within the range on the floor.
+ * They should have ensured that items in this range won't be dirty.
+ */
+void scoutfs_item_invalidate(struct super_block *sb,
+			     struct scoutfs_key_buf *start,
+			     struct scoutfs_key_buf *end)
+{
+	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
+	struct item_cache *cac = sbi->item_cache;
+	struct cached_item *next;
+	struct cached_item *item;
+	struct rb_node *node;
+	unsigned long flags;
+
+	/* XXX think about racing with trans write */
+
+	spin_lock_irqsave(&cac->lock, flags);
+
+	for (item = next_item(&cac->items, start);
+	     item && scoutfs_key_compare(item->key, end) <= 0;
+	     item = next) {
+
+		/* XXX seems like this should be a helper? */
+		node = rb_next(&item->node);
+		if (node)
+			next = container_of(node, struct cached_item, node);
+		else
+			next = NULL;
+
+		WARN_ON_ONCE(item->dirty & ITEM_DIRTY);
+		erase_item(sb, cac, item);
+	}
+
+	spin_unlock_irqrestore(&cac->lock, flags);
+}
+
 int scoutfs_item_setup(struct super_block *sb)
 {
 	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
diff --git a/kmod/src/item.h b/kmod/src/item.h
index bd3e3f73..fced5957 100644
--- a/kmod/src/item.h
+++ b/kmod/src/item.h
@@ -57,6 +57,12 @@ bool scoutfs_item_has_dirty(struct super_block *sb);
 bool scoutfs_item_dirty_fits_single(struct super_block *sb, u32 nr_items,
 			            u32 key_bytes, u32 val_bytes);
 int scoutfs_item_dirty_seg(struct super_block *sb, struct scoutfs_segment *seg);
+int scoutfs_item_writeback(struct super_block *sb,
+			   struct scoutfs_key_buf *start,
+			   struct scoutfs_key_buf *end);
+void scoutfs_item_invalidate(struct super_block *sb,
+			     struct scoutfs_key_buf *start,
+			     struct scoutfs_key_buf *end);
 
 int scoutfs_item_setup(struct super_block *sb);
 void scoutfs_item_destroy(struct super_block *sb);
diff --git a/kmod/src/lock.c b/kmod/src/lock.c
new file mode 100644
index 00000000..62f09129
--- /dev/null
+++ b/kmod/src/lock.c
@@ -0,0 +1,282 @@
+/*
+ * Copyright (C) 2018 Versity Software, Inc.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * 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
+ * General Public License for more details.
+ */
+#include <linux/kernel.h>
+#include <linux/fs.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+
+#include "super.h"
+#include "lock.h"
+#include "item.h"
+#include "scoutfs_trace.h"
+
+/*
+ * This is meant to be simple and correct, not performant.
+ */
+
+static DECLARE_RWSEM(global_rwsem);
+static LIST_HEAD(global_super_list);
+
+/*
+ * Allocated once and pointed to by the lock info of all the supers with
+ * the same fsid.  Freed as the last super unmounts.
+ */
+struct held_locks {
+	spinlock_t lock;
+	struct list_head list;
+	wait_queue_head_t waitq;
+};
+
+
+/*
+ * allocated per-super.  Stored in the global list for finding supers
+ * with fsids and stored in a list with others with the same fsid for
+ * invalidation.  Freed on unmount.
+ */
+struct lock_info {
+	struct super_block *sb;
+	struct held_locks *held;
+	struct list_head id_head;
+	struct list_head global_head;
+};
+
+#define DECLARE_LOCK_INFO(sb, name) \
+	struct lock_info *name = SCOUTFS_SB(sb)->lock_info
+
+/*
+ * locks are compatible if they're from the same super, or are both reads,
+ * or don't overlap.
+ */
+static bool compatible_locks(struct scoutfs_lock *a, struct scoutfs_lock *b)
+{
+	return a->sb == b->sb ||
+	       (a->mode == SCOUTFS_LOCK_MODE_READ &&
+	        b->mode == SCOUTFS_LOCK_MODE_READ) ||
+	       scoutfs_key_compare_ranges(a->start, a->end, b->start, b->end);
+}
+
+static bool lock_added(struct held_locks *held, struct scoutfs_lock *add)
+{
+	struct scoutfs_lock *lck;
+	bool added = true;
+
+	spin_lock(&held->lock);
+
+	list_for_each_entry(lck, &held->list, head) {
+		if (!compatible_locks(lck, add)) {
+			added = false;
+			break;
+		}
+	}
+
+	if (added)
+		list_add(&add->head, &held->list);
+
+	spin_unlock(&held->lock);
+
+	return added;
+}
+
+/*
+ * Invalidate caches on this super because another super has acquired
+ * a lock with the given mode and range.  We always have to write out
+ * dirty overlapping items.  If they're writing then we need to also
+ * invalidate all cached overlapping structures.
+ */
+static int invalidate_caches(struct super_block *sb, int mode,
+			     struct scoutfs_key_buf *start,
+			     struct scoutfs_key_buf *end)
+{
+	int ret;
+
+	ret = scoutfs_item_writeback(sb, start, end);
+	if (ret)
+		return ret;
+
+	if (mode == SCOUTFS_LOCK_MODE_WRITE) {
+		scoutfs_item_invalidate(sb, start, end);
+#if 0
+		scoutfs_dir_invalidate(sb, start, end) ?:
+		scoutfs_inode_invalidate(sb, start, end) ?:
+		scoutfs_data_invalidate(sb, start, end);
+#endif
+	}
+
+	return 0;
+}
+
+#define for_each_other_linf(linf, from_linf)				  \
+	for (linf = list_entry(from_linf->id_head.next, struct lock_info, \
+			       id_head);				  \
+	     linf != from_linf;						  \
+	     linf = list_entry(linf->id_head.next, struct lock_info,	  \
+			       id_head))
+
+static int invalidate_others(struct super_block *from, int mode,
+			     struct scoutfs_key_buf *start,
+			     struct scoutfs_key_buf *end)
+{
+	DECLARE_LOCK_INFO(from, from_linf);
+	struct lock_info *linf;
+	int ret;
+
+	down_read(&global_rwsem);
+
+	for_each_other_linf(linf, from_linf) {
+		ret = invalidate_caches(linf->sb, mode, start, end);
+		if (ret)
+			break;
+	}
+
+	up_read(&global_rwsem);
+
+	return ret;
+}
+
+static void unlock(struct held_locks *held, struct scoutfs_lock *lck)
+{
+	spin_lock(&held->lock);
+	list_del_init(&lck->head);
+	spin_unlock(&held->lock);
+
+	wake_up(&held->waitq);
+}
+
+/*
+ * Acquire a coherent lock on the given range of keys.  While the lock
+ * is held other lockers are serialized.  Cache coherency is maintained
+ * by the locking infrastructure.  Lock acquisition causes writeout from
+ * or invalidation of other caches.
+ *
+ * The caller provides the opaque lock structure used for storage and
+ * their start and end pointers will be accessed while the lock is held.
+ */
+int scoutfs_lock_range(struct super_block *sb, int mode,
+		       struct scoutfs_key_buf *start,
+		       struct scoutfs_key_buf *end,
+		       struct scoutfs_lock *lck)
+{
+	DECLARE_LOCK_INFO(sb, linf);
+	struct held_locks *held = linf->held;
+	int ret;
+
+	INIT_LIST_HEAD(&lck->head);
+	lck->sb = sb;
+	lck->start = start;
+	lck->end = end;
+	lck->mode = mode;
+
+	ret = wait_event_interruptible(held->waitq, lock_added(held, lck));
+	if (ret == 0) {
+		ret = invalidate_others(sb, mode, start, end);
+		if (ret)
+			unlock(held, lck);
+	}
+
+	return ret;
+}
+
+void scoutfs_unlock_range(struct super_block *sb, struct scoutfs_lock *lck)
+{
+	DECLARE_LOCK_INFO(sb, linf);
+	struct held_locks *held = linf->held;
+
+	unlock(held, lck);
+}
+
+/*
+ * The moment this is done we can have other mounts start asking
+ * us to write back and invalidate, so do this very very late.
+ */
+int scoutfs_lock_setup(struct super_block *sb)
+{
+	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
+	struct scoutfs_sb_info *other_sbi;
+	struct lock_info *other_linf;
+	struct held_locks *held;
+	struct lock_info *linf;
+
+	linf = kmalloc(sizeof(struct lock_info), GFP_KERNEL);
+	if (!linf)
+		return -ENOMEM;
+
+	held = kmalloc(sizeof(struct held_locks), GFP_KERNEL);
+	if (!held) {
+		kfree(linf);
+		return -ENOMEM;
+	}
+
+	spin_lock_init(&held->lock);
+	INIT_LIST_HEAD(&held->list);
+	init_waitqueue_head(&held->waitq);
+
+	linf->sb = sb;
+	linf->held = held;
+	INIT_LIST_HEAD(&linf->id_head);
+	INIT_LIST_HEAD(&linf->global_head);
+
+	sbi->lock_info = linf;
+
+	trace_printk("sb %p id %016llx allocated linf %p held %p\n",
+		     sb, le64_to_cpu(sbi->super.id), linf, held);
+
+	down_write(&global_rwsem);
+
+	list_for_each_entry(other_linf, &global_super_list, global_head) {
+		other_sbi = SCOUTFS_SB(other_linf->sb);
+		if (other_sbi->super.id == sbi->super.id) {
+			list_add(&linf->id_head, &other_linf->id_head);
+			linf->held = other_linf->held;
+			trace_printk("sharing held %p\n", linf->held);
+			break;
+		}
+	}
+
+	/* add to global list after walking so we don't see ourselves */
+	list_add(&linf->global_head, &global_super_list);
+
+	up_write(&global_rwsem);
+
+	if (linf->held != held)
+		kfree(held);
+
+	return 0;
+}
+
+void scoutfs_lock_destroy(struct super_block *sb)
+{
+	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
+	DECLARE_LOCK_INFO(sb, linf);
+	struct held_locks *held;
+
+	if (linf) {
+		down_write(&global_rwsem);
+
+		list_del_init(&linf->global_head);
+
+		if (!list_empty(&linf->id_head)) {
+			list_del_init(&linf->id_head);
+			held = NULL;
+		} else {
+			held = linf->held;
+		}
+
+		up_write(&global_rwsem);
+
+		trace_printk("sb %p id %016llx freeing linf %p held %p\n",
+			     sb, le64_to_cpu(sbi->super.id), linf, held);
+
+		kfree(held);
+		kfree(linf);
+	}
+}
diff --git a/kmod/src/lock.h b/kmod/src/lock.h
new file mode 100644
index 00000000..1f7d2681
--- /dev/null
+++ b/kmod/src/lock.h
@@ -0,0 +1,26 @@
+#ifndef _SCOUTFS_LOCK_H_
+#define _SCOUTFS_LOCK_H_
+
+struct scoutfs_lock {
+	struct list_head head;
+	struct super_block *sb;
+	struct scoutfs_key_buf *start;
+	struct scoutfs_key_buf *end;
+	int mode;
+};
+
+enum {
+	SCOUTFS_LOCK_MODE_READ,
+	SCOUTFS_LOCK_MODE_WRITE,
+};
+
+int scoutfs_lock_range(struct super_block *sb, int mode,
+		       struct scoutfs_key_buf *start,
+		       struct scoutfs_key_buf *end,
+		       struct scoutfs_lock *lck);
+void scoutfs_unlock_range(struct super_block *sb, struct scoutfs_lock *lck);
+
+int scoutfs_lock_setup(struct super_block *sb);
+void scoutfs_lock_destroy(struct super_block *sb);
+
+#endif
diff --git a/kmod/src/super.c b/kmod/src/super.c
index 158052c1..4aa4d28f 100644
--- a/kmod/src/super.c
+++ b/kmod/src/super.c
@@ -35,6 +35,7 @@
 #include "treap.h"
 #include "compact.h"
 #include "data.h"
+#include "lock.h"
 #include "scoutfs_trace.h"
 
 static struct kset *scoutfs_kset;
@@ -219,7 +220,8 @@ static int scoutfs_fill_super(struct super_block *sb, void *data, int silent)
 	      scoutfs_treap_setup(sb) ?:
 //	      scoutfs_buddy_setup(sb) ?:
 	      scoutfs_compact_setup(sb) ?:
-	      scoutfs_setup_trans(sb);
+	      scoutfs_setup_trans(sb) ?:
+	      scoutfs_lock_setup(sb);
 	if (ret)
 		return ret;
 
@@ -250,6 +252,7 @@ static void scoutfs_kill_sb(struct super_block *sb)
 
 	kill_block_super(sb);
 	if (sbi) {
+		scoutfs_lock_destroy(sb);
 		scoutfs_compact_destroy(sb);
 		scoutfs_shutdown_trans(sb);
 		scoutfs_data_destroy(sb);
diff --git a/kmod/src/super.h b/kmod/src/super.h
index e791e76d..5f1b468d 100644
--- a/kmod/src/super.h
+++ b/kmod/src/super.h
@@ -13,6 +13,7 @@ struct segment_cache;
 struct treap_info;
 struct compact_info;
 struct data_info;
+struct lock_info;
 
 struct scoutfs_sb_info {
 	struct super_block *sb;
@@ -40,6 +41,8 @@ struct scoutfs_sb_info {
 	wait_queue_head_t trans_write_wq;
 	struct workqueue_struct *trans_write_workq;
 
+	struct lock_info *lock_info;
+
 	/* $sysfs/fs/scoutfs/$id/ */
 	struct kset *kset;