Date: Tue, 25 Jun 2024 22:17:57 +0100
From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
To: linux-kernel@...r.kernel.org
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	linux-fsdevel@...r.kernel.org,
	maple-tree@...ts.infradead.org
Subject: [PATCH v2 2/5] rosebush: Add new data structure

Rosebush is a resizing hash table.  See
Docuemntation/core-api/rosebush.rst for details.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 Documentation/core-api/index.rst    |   1 +
 Documentation/core-api/rosebush.rst | 121 +++++
 MAINTAINERS                         |   8 +
 include/linux/rosebush.h            |  62 +++
 lib/Makefile                        |   2 +-
 lib/rosebush.c                      | 654 ++++++++++++++++++++++++++++
 6 files changed, 847 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/core-api/rosebush.rst
 create mode 100644 include/linux/rosebush.h
 create mode 100644 lib/rosebush.c

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f147854700e4..8984a7d6ad73 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -36,6 +36,7 @@ Library functionality that is used throughout the kernel.
    kobject
    kref
    assoc_array
+   rosebush
    xarray
    maple_tree
    idr
diff --git a/Documentation/core-api/rosebush.rst b/Documentation/core-api/rosebush.rst
new file mode 100644
index 000000000000..f0c0bc690e48
--- /dev/null
+++ b/Documentation/core-api/rosebush.rst
@@ -0,0 +1,121 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+========
+Rosebush
+========
+
+:Author: Matthew Wilcox
+
+Overview
+========
+
+Rosebush is a hashtable, different from the rhashtable.  It is scalable
+(one spinlock per bucket), resizing in two dimensions (number and size
+of buckets), and concurrent (can be iterated under the RCU read lock).
+It is designed to minimise dependent cache misses, which can stall a
+modern CPU for thousands of instructions.
+
+Objects stored in a rosebush do not have an embedded linked list.
+They can therefore be placed into the same rosebush multiple times and
+be placed in multiple rosebushes.  It is also possible to store pointers
+which have special meaning like ERR_PTR().  It is not possible to store
+a NULL pointer in a rosebush, as this is the return value that indicates
+the iteration has finished.
+
+The user of the rosebush is responsible for calculating their own hash.
+A high quality hash is desirable to keep the scalable properties of
+the rosebush, but a hash with poor distribution in the lower bits will
+not lead to a catastrophic breakdown.  It may lead to excessive memory
+consumption and a lot of CPU time spent during lookup.
+
+Rosebush is not yet IRQ or BH safe.  It can be iterated in interrupt
+context, but not modified.
+
+RCU Iteration
+-------------
+
+There is no rosebush_lookup() function.  This is because the hash value
+may not be unique.  Instead, the user should iterate the rosebush,
+which will return pointers which probably have matching hash values.
+It is the user's responsibility to determine if the returned pointer is
+one they are interested in.
+
+Rosebush iteration guarantees to return all pointers which have a
+matching hash, were in the rosebush before the iteration started and
+remain in the rosebush after iteration ends.  It may return additional
+pointers, including pointers which do not have a matching hash value,
+but it guarantees not to skip any pointers, and it guarantees to only
+return pointers which have (at some point) been stored in the rosebush.
+
+If the rosebush is modified while the iteration is in progress, newly
+added entries may or may not be returned and removed entries may or may
+not be returned.  Causality is not honoured; e.g. if Entry A is known
+to be inserted before Entry B, it is possible for an iteration to return
+Entry B and not Entry A.
+
+Functions and structures
+========================
+
+.. kernel-doc:: include/linux/rosebush.h
+.. kernel-doc:: lib/rosebush.c
+
+Internals
+=========
+
+The rosebush is organised into a top level table which contains pointers
+to buckets.  Each bucket contains a spinlock (for modifications to the
+bucket), the number of entries in the bucket and a contention counter.
+
+The top level table is a power of two in size.  The bottom M bits of the
+hash are used to index into this table.  The bucket contains hash values
+followed by their corresponding pointers.  We also track a contention
+count, which lets us know if this spinlock is overloaded and we should
+split the bucket to improve scalability.
+
+A bucket may be shared between multiple table entries.  For simplicity,
+we require that all buckets are shared between a power-of-two number
+of slots.  For example, a table with 8 entries might have entries that
+point to buckets A, B, A, B, A, C, A, D.  If we were to then split bucket
+A, we would replace the first pair of pointers with pointers to bucket
+E and the second pair with pointers to bucket F.  This is akin to the
+buddy page allocator.
+
+When we decide that the table needs to be resized, we allocate a new
+table, and duplicate the current contents of the table into each half
+of the new table.  At this point, all buckets in the table are shared.
+We then split the bucket that we're trying to insert into.
+
+IRQ / BH safety
+---------------
+
+If we decide to make the rosebush modifiable in IRQ context, we need
+to take the locks in an irq-safe way; we need to figure out how to
+allocate the top level table without vmalloc(), and we need to manage
+without kvfree_rcu_mightsleep().  These all have solutions, but those
+solutions have a cost that isn't worth paying until we have users.
+
+Some of those problems go away if we limit our support to removal in IRQ
+context and only allow insertions in process context (as we do not need
+to reallocate the table or bucket when removing an item).
+
+Small rosebushes
+----------------
+
+As an optimisation, if the rosebush has no entries, the buckets pointer
+is NULL.  If the rosebush has only a few entries, there are only two
+buckets (allocated as a single allocation) and the table pointer points
+directly to the first one instead of pointing to a table.
+
+Shrinking the rosebush
+----------------------
+
+We do not currently attempt either to join buckets or to shrink the table
+if sufficiently many buckets are shared.  If this proves to be a desirable
+course of action, we can add support for it, with sufficient hysteresis
+that adding/removing a single entry will not cause bouncing.
+
+Rosebush statistics
+-------------------
+
+It would probably be wise to be able to gather statistics about bucket
+occupancy rates, but this work has not yet been done.
diff --git a/MAINTAINERS b/MAINTAINERS
index a39c237edb95..d4a8e99919a4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -19467,6 +19467,14 @@ F:	include/net/rose.h
 F:	include/uapi/linux/rose.h
 F:	net/rose/
 
+ROSEBUSH DATA STRUCTURE
+M:	Matthew Wilcox <willy@...radead.org>
+L:	maple-tree@...ts.infradead.org
+S:	Supported
+F:	Documentation/core-api/rosebush.rst
+F:	include/linux/rosebush.h
+F:	lib/*rosebush.c
+
 ROTATION DRIVER FOR ALLWINNER A83T
 M:	Jernej Skrabec <jernej.skrabec@...il.com>
 L:	linux-media@...r.kernel.org
diff --git a/include/linux/rosebush.h b/include/linux/rosebush.h
new file mode 100644
index 000000000000..57f4dfa3f93d
--- /dev/null
+++ b/include/linux/rosebush.h
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* See lib/rosebush.c */
+
+#ifndef _LINUX_ROSEBUSH_H
+#define _LINUX_ROSEBUSH_H
+
+#include <linux/spinlock.h>
+
+/*
+ * Embed this struct in your struct, don't allocate it separately.
+ * None of this is for customers to use; treat it as opaque.
+ * In particular, taking the rbh_resize_lock will prevent only rbh_table
+ * from being reallocated; buckets can still be grown and split without
+ * the lock.  But you will get incomprehensible lockdep warnings!
+ */
+struct rbh {
+	spinlock_t	rbh_resize_lock;
+	unsigned long	rbh_table;	/* A tagged pointer */
+};
+
+#define DEFINE_ROSEBUSH(name)	struct rbh name = \
+	{ .rbh_resize_lock = __SPIN_LOCK_UNLOCKED(name.lock), }
+
+int rbh_insert(struct rbh *rbh, u32 hash, void *p);
+int rbh_reserve(struct rbh *rbh, u32 hash);
+int rbh_use(struct rbh *rbh, u32 hash, void *p);
+int rbh_remove(struct rbh *rbh, u32 hash, void *p);
+void rbh_destroy(struct rbh *rbh);
+
+/**
+ * rbh_release - Release a reserved slot in a rosebush.
+ * @rbh: The rosebush.
+ * @hash: The hash value that was reserved.
+ *
+ * If you decide that you do not need to use a reserved slot, call this
+ * function to release the slot.
+ *
+ * Return: 0 on success, -ENOENT if no reserved slot was found.
+ */
+
+static inline int rbh_release(struct rbh *rbh, u32 hash)
+{
+	return rbh_remove(rbh, hash, NULL);
+}
+
+struct rbh_iter {
+	struct rbh *rbh;
+	struct rbh_bucket *bucket;
+	u32 hash;
+	unsigned int index;
+};
+
+#define RBH_ITER(name, _rbh, _hash)					\
+	struct rbh_iter name = { .rbh = _rbh, .hash = _hash, }
+
+void *rbh_next(struct rbh_iter *rbhi);
+
+void rbh_iter_remove(struct rbh_iter *rbhi, void *p);
+void rbh_iter_lock(struct rbh_iter *rbhi);
+void rbh_iter_unlock(struct rbh_iter *rbhi);
+
+#endif
diff --git a/lib/Makefile b/lib/Makefile
index 3b1769045651..723e6c90b58d 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -28,7 +28,7 @@ CFLAGS_string.o += -fno-stack-protector
 endif
 
 lib-y := ctype.o string.o vsprintf.o cmdline.o \
-	 rbtree.o radix-tree.o timerqueue.o xarray.o \
+	 rosebush.o rbtree.o radix-tree.o timerqueue.o xarray.o \
 	 maple_tree.o idr.o extable.o irq_regs.o argv_split.o \
 	 flex_proportions.o ratelimit.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
diff --git a/lib/rosebush.c b/lib/rosebush.c
new file mode 100644
index 000000000000..47106a04d11d
--- /dev/null
+++ b/lib/rosebush.c
@@ -0,0 +1,654 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Rosebush, a resizing bucket hash table
+ * Copyright (c) 2024 Oracle Corporation
+ * Author: Matthew Wilcox <willy@...radead.org>
+ */
+
+#include <linux/export.h>
+#include <linux/rcupdate.h>
+#include <linux/rosebush.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/stddef.h>
+#include <linux/vmalloc.h>
+
+#include <asm/barrier.h>
+
+/*
+ * The lock is held whenever we are modifying the contents of the bucket.
+ * The contention counter tracks whether we need to split the bucket due
+ * to contention on the spinlock.
+ * The bucket is 256 bytes in size (20 * 12 = 240, plus parent, lock, etc)
+ */
+#ifdef CONFIG_64BIT
+#define RBH_ENTRIES	20
+#else
+#define RBH_ENTRIES	30
+#endif
+
+struct rbh_bucket {
+	u32 rbh_hashes[RBH_ENTRIES];
+	void __rcu *rbh_slots[RBH_ENTRIES];
+	const struct rbh *rbh;
+	spinlock_t rbh_lock;
+	u32 rbh_contention;
+};
+
+struct rbh_table {
+	DECLARE_FLEX_ARRAY(struct rbh_bucket __rcu *, buckets);
+};
+
+struct rbh_initial_table {
+	struct rbh_bucket buckets[2];
+};
+
+/*
+ * As far as lockdep is concerned, all buckets in the same rosebush use
+ * the same lock.  We use the classes to distinguish the rbh resize lock
+ * from the bucket locks.  The only time we take two bucket locks is
+ * when we already hold the resize lock, so there is no need to define
+ * an order between bucket locks.
+ */
+#ifdef CONFIG_DEBUG_SPINLOCK
+#define bucket_lock_init(rbh, bucket)					\
+	__raw_spin_lock_init(spinlock_check(&(bucket)->rbh_lock),	\
+		"rbh", (rbh)->rbh_resize_lock.dep_map.key, LD_WAIT_SPIN)
+#else
+#define bucket_lock_init(rbh, bucket)					\
+	spin_lock_init(&(bucket)->rbh_lock)
+#endif
+
+#define rbh_resize_lock(rbh)	spin_lock_nested(&(rbh)->rbh_resize_lock, 2)
+#define rbh_resize_unlock(rbh)	spin_unlock(&(rbh)->rbh_resize_lock)
+
+struct table_mask {
+	struct rbh_table *table;
+	u32 mask;
+};
+
+/*
+ * A very complicated way of spelling &rbh->bucket[hash].
+ *
+ * The first complication is that we encode the number of buckets
+ * in the pointer so that we can get both in an atomic load.
+ *
+ * The second complication is that small hash tables don't have a top
+ * level table; instead the buckets pointer points to a pair of buckets.
+ */
+static struct rbh_bucket *get_bucket(struct rbh *rbh, u32 hash,
+		struct table_mask *state)
+{
+	unsigned long tagged;
+	struct rbh_initial_table *initial_table;
+	unsigned int bnr;
+
+	/* rcu_dereference(), but not a pointer */
+	tagged = READ_ONCE(rbh->rbh_table);
+	if (!tagged)
+		return NULL;
+
+	/* The lowest bits indicates how many buckets the table holds */
+	state->table = (struct rbh_table *)(tagged & (tagged + 1));
+	state->mask = tagged - (unsigned long)state->table;
+	bnr = hash & state->mask;
+	if (state->mask != 1)
+		return rcu_dereference(state->table->buckets[bnr]);
+
+	initial_table = (struct rbh_initial_table *)state->table;
+	return &initial_table->buckets[bnr];
+}
+
+static struct rbh_bucket *lock_bucket(struct rbh *rbh, u32 hash)
+	__acquires(&bucket->rbh_lock)
+{
+	struct rbh_bucket *bucket, *new_bucket;
+	struct table_mask state;
+
+	bucket = get_bucket(rbh, hash, &state);
+	if (!bucket)
+		return bucket;
+again:
+	spin_lock(&bucket->rbh_lock);
+	new_bucket = get_bucket(rbh, hash, &state);
+	if (bucket == new_bucket)
+		return bucket;
+	spin_unlock(&bucket->rbh_lock);
+	bucket = new_bucket;
+	goto again;
+}
+
+static bool rbh_first(struct rbh *rbh, u32 hash)
+{
+	struct rbh_initial_table *table;
+	int i;
+
+//printk("%s: table size %zd\n", __func__, sizeof(*table));
+	table = kmalloc(sizeof(*table), GFP_KERNEL);
+	if (!table)
+		return false;
+
+	rbh_resize_lock(rbh);
+	if (rbh->rbh_table) {
+		rbh_resize_unlock(rbh);
+		/* Somebody else resized it for us */
+		kfree(table);
+		return true;
+	}
+
+	bucket_lock_init(rbh, &table->buckets[0]);
+	table->buckets[0].rbh = rbh;
+	table->buckets[0].rbh_contention = 0;
+	bucket_lock_init(rbh, &table->buckets[1]);
+	table->buckets[1].rbh = rbh;
+	table->buckets[1].rbh_contention = 0;
+	for (i = 0; i < RBH_ENTRIES; i++) {
+		table->buckets[0].rbh_hashes[i] = ~0;
+		table->buckets[1].rbh_hashes[i] = 0;
+	}
+	/* rcu_assign_pointer() but not a pointer */
+	smp_store_release(&rbh->rbh_table, (unsigned long)table | 1);
+	rbh_resize_unlock(rbh);
+
+//printk("%s: new table = %px\n", __func__, table);
+	return true;
+}
+
+static void copy_initial_buckets(const struct rbh *rbh,
+		struct rbh_table *table, struct rbh_initial_table *init_table)
+	__acquires(&init_table->buckets[0].rbh_lock)
+	__acquires(&init_table->buckets[1].rbh_lock)
+{
+	struct rbh_bucket *bucket;
+
+	bucket = (void __force *)table->buckets[0];
+	spin_lock(&init_table->buckets[0].rbh_lock);
+	memcpy(bucket, &init_table->buckets[0], sizeof(init_table->buckets[0]));
+	bucket_lock_init(rbh, bucket);
+
+	bucket = (void __force *)table->buckets[1];
+	spin_lock_nested(&init_table->buckets[1].rbh_lock, 1);
+	memcpy(bucket, &init_table->buckets[1], sizeof(init_table->buckets[1]));
+	bucket_lock_init(rbh, bucket);
+}
+
+/*
+ * When we grow the table, we duplicate the bucket pointers so this
+ * thread doesn't pay the entire cost of growing the table.
+ */
+static int rbh_grow_table(struct rbh *rbh, u32 hash, struct table_mask *state)
+	__releases(RCU)
+	__acquires(RCU)
+{
+	struct rbh_table *table;
+	struct rbh_initial_table *init_table;
+	u32 mask = state->mask;
+	unsigned long tagged = (unsigned long)state->table | mask;
+	size_t size;
+
+	rcu_read_unlock();
+
+	size = (mask + 1) * 2 * sizeof(void *);
+	if (size > 4 * PAGE_SIZE)
+		/* XXX: NUMA_NO_NODE doesn't necessarily interleave */
+		table = __vmalloc_node(size, size, GFP_KERNEL, NUMA_NO_NODE,
+				&table);
+	else
+		table = kvmalloc(size, GFP_KERNEL);
+	if (!table) {
+		rcu_read_lock();
+		/* Maybe somebody resized it for us */
+		if (READ_ONCE(rbh->rbh_table) != tagged)
+			return 0;
+		return -ENOMEM;
+	}
+	BUG_ON((unsigned long)table & (size - 1));
+
+	if (mask == 1) {
+		/* Don't need to bother with RCU until we publish the table */
+		table->buckets[0] = (void __rcu *)kmalloc(sizeof(struct rbh_bucket), GFP_KERNEL);
+		if (!table->buckets[0])
+			goto free_all;
+		table->buckets[1] = (void __rcu *)kmalloc(sizeof(struct rbh_bucket), GFP_KERNEL);
+		if (!table->buckets[1])
+			goto free_all;
+	}
+
+	rbh_resize_lock(rbh);
+	if (rbh->rbh_table != tagged) {
+		rbh_resize_unlock(rbh);
+		/* Somebody else resized it for us */
+		kvfree(table);
+		rcu_read_lock();
+		return 0;
+	}
+
+//printk("%s: replacing table %p with table %p mask %d\n", __func__, state->table, table, mask);
+	if (mask == 1) {
+		init_table = (void *)state->table;
+		copy_initial_buckets(rbh, table, init_table);
+	} else {
+		memcpy(&table->buckets, &state->table->buckets,
+				(mask + 1) * sizeof(void *));
+	}
+	memcpy(&table->buckets[mask + 1], &table->buckets[0],
+			(mask + 1) * sizeof(void *));
+
+	tagged = ((unsigned long)table) | (mask << 1) | 1;
+	/* rcu_assign_pointer() but not a pointer */
+	smp_store_release(&rbh->rbh_table, tagged);
+	rbh_resize_unlock(rbh);
+	if (mask == 1) {
+		spin_unlock(&init_table->buckets[0].rbh_lock);
+		spin_unlock(&init_table->buckets[1].rbh_lock);
+	}
+	kvfree_rcu_mightsleep(state->table);
+
+	rcu_read_lock();
+	return 0;
+free_all:
+	kfree((void __force *)table->buckets[0]);
+	kvfree(table);
+	rcu_read_lock();
+	return -ENOMEM;
+}
+
+static void bucket_copy(const struct rbh *rbh, struct rbh_bucket *buckets[2],
+		const struct rbh_bucket *old_bucket, u32 hash, u32 mask)
+{
+	unsigned int i;
+	unsigned int js[2] = {0, 0};
+
+	bucket_lock_init(rbh, buckets[0]);
+	buckets[0]->rbh_contention = 0;
+	buckets[0]->rbh = rbh;
+	bucket_lock_init(rbh, buckets[1]);
+	buckets[1]->rbh_contention = 0;
+	buckets[1]->rbh = rbh;
+	for (i = 0; i < RBH_ENTRIES; i++) {
+		unsigned int nr = !!(old_bucket->rbh_hashes[i] & mask);
+		buckets[nr]->rbh_hashes[js[nr]] = old_bucket->rbh_hashes[i];
+		buckets[nr]->rbh_slots[js[nr]++] = old_bucket->rbh_slots[i];
+	}
+//printk("%s: bucket:%p copied %d entries from %p hash:%x mask:%x\n", __func__, buckets[0], js[0], old_bucket, hash, mask);
+//printk("%s: bucket:%p copied %d entries from %p hash:%x mask:%x\n", __func__, buckets[1], js[1], old_bucket, hash, mask);
+
+	/* Fill the new buckets with deleted entries */
+	while (js[0] < RBH_ENTRIES)
+		buckets[0]->rbh_hashes[js[0]++] = ~hash;
+	while (js[1] < RBH_ENTRIES)
+		buckets[1]->rbh_hashes[js[1]++] = ~hash;
+}
+
+#define rbh_dereference_protected(p, rbh)				\
+	rcu_dereference_protected(p, lockdep_is_held(&(rbh)->rbh_resize_lock))
+
+static int rbh_split_bucket(struct rbh *rbh, struct rbh_bucket *bucket,
+		u32 hash, struct table_mask *state)
+{
+	unsigned long tagged;
+	u32 bit, mask;
+	struct rbh_table *table;
+	struct rbh_bucket *buckets[2];
+	int err = 0;
+
+	if (state->mask == 1) {
+		err = rbh_grow_table(rbh, hash, state);
+	} else {
+		u32 buddy = (hash & state->mask) ^ ((state->mask + 1) / 2);
+		if (rcu_dereference(state->table->buckets[buddy]) != bucket)
+			err = rbh_grow_table(rbh, hash, state);
+	}
+	if (err < 0)
+		return err;
+
+	rcu_read_unlock();
+
+	/* XXX: use slab */
+	buckets[0] = kmalloc(sizeof(*bucket), GFP_KERNEL);
+	if (!buckets[0])
+		return -ENOMEM;
+	buckets[1] = kmalloc(sizeof(*bucket), GFP_KERNEL);
+	if (!buckets[1]) {
+		kfree(buckets[0]);
+		return -ENOMEM;
+	}
+
+//printk("%s: adding buckets %p %p for hash %d\n", __func__, buckets[0], buckets[1], hash);
+	rbh_resize_lock(rbh);
+	tagged = rbh->rbh_table;
+	table = (struct rbh_table *)(tagged & (tagged + 1));
+	mask = tagged - (unsigned long)table;
+	hash &= mask;
+	if (rbh_dereference_protected(table->buckets[hash], rbh) != bucket)
+		goto free;
+
+	/* Figure out which entries we need to take to the new bucket */
+	bit = mask + 1;
+	while (bit > 1) {
+		bit /= 2;
+//printk("hash:%d buddy:%d\n", hash, hash ^ bit);
+		if (rbh_dereference_protected(table->buckets[hash ^ bit], rbh)
+				!= bucket)
+			break;
+	}
+	bit *= 2;
+	if (bit == mask + 1)
+		goto free;
+
+	spin_lock(&bucket->rbh_lock);
+	bucket_copy(rbh, buckets, bucket, hash, bit);
+
+//printk("hash:%d mask:%d bit:%d\n", hash, mask, bit);
+	hash &= (bit - 1);
+	do {
+//printk("assigning bucket %p to index %d\n", buckets[0], hash);
+		rcu_assign_pointer(table->buckets[hash], buckets[0]);
+		hash += bit;
+//printk("assigning bucket %p to index %d\n", buckets[1], hash);
+		rcu_assign_pointer(table->buckets[hash], buckets[1]);
+		hash += bit;
+	} while (hash < mask);
+//printk("owner:%px\n", bucket->rbh_lock.owner)
+	spin_unlock(&bucket->rbh_lock);
+	rbh_resize_unlock(rbh);
+	kvfree_rcu_mightsleep(bucket);
+
+	return 0;
+free:
+	rbh_resize_unlock(rbh);
+//printk("%s: freeing bucket %p\n", __func__, bucket);
+	kfree(buckets[0]);
+	kfree(buckets[1]);
+
+	return 0;
+}
+
+static int __rbh_insert(struct rbh *rbh, u32 hash, void *p)
+{
+	struct table_mask state;
+	struct rbh_bucket *bucket, *new_bucket;
+	unsigned int i;
+	int err;
+
+restart:
+	rcu_read_lock();
+	bucket = get_bucket(rbh, hash, &state);
+	if (unlikely(!bucket)) {
+		rcu_read_unlock();
+		if (!rbh_first(rbh, hash))
+			return -ENOMEM;
+		goto restart;
+	}
+
+again:
+	if (spin_trylock(&bucket->rbh_lock)) {
+		if (bucket->rbh_contention)
+			bucket->rbh_contention--;
+	} else {
+		spin_lock(&bucket->rbh_lock);
+		/* Numbers chosen ad-hoc */
+		bucket->rbh_contention += 10;
+		if (unlikely(bucket->rbh_contention > 5000)) {
+			spin_unlock(&bucket->rbh_lock);
+			/* OK if this fails; it's only contention */
+			rbh_split_bucket(rbh, bucket, hash, &state);
+
+			bucket = get_bucket(rbh, hash, &state);
+			spin_lock(&bucket->rbh_lock);
+		}
+	}
+
+	new_bucket = get_bucket(rbh, hash, &state);
+	if (bucket != new_bucket) {
+		spin_unlock(&bucket->rbh_lock);
+		bucket = new_bucket;
+		goto again;
+	}
+
+	/* Deleted elements differ in their bottom bit */
+	for (i = 0; i < RBH_ENTRIES; i++) {
+		u32 bhash = bucket->rbh_hashes[i];
+
+		if ((bhash & 1) == (hash & 1))
+			continue;
+		rcu_assign_pointer(bucket->rbh_slots[i], p);
+		/* This array is read under RCU */
+		WRITE_ONCE(bucket->rbh_hashes[i], hash);
+
+		spin_unlock(&bucket->rbh_lock);
+		rcu_read_unlock();
+		return 0;
+	}
+
+	/* No space in this bucket */
+	spin_unlock(&bucket->rbh_lock);
+
+	err = rbh_split_bucket(rbh, bucket, hash, &state);
+	rcu_read_unlock();
+	if (err)
+		return err;
+	goto restart;
+}
+
+/**
+ * rbh_insert - Add a pointer to a rosebush.
+ * @rbh: The rosebush.
+ * @hash: The hash value for this pointer.
+ * @p: The pointer to add.
+ *
+ * Return: 0 on success, -ENOMEM if memory allocation fails,
+ * -EINVAL if @p is NULL.
+ */
+int rbh_insert(struct rbh *rbh, u32 hash, void *p)
+{
+	if (p == NULL)
+		return -EINVAL;
+	return __rbh_insert(rbh, hash, p);
+}
+EXPORT_SYMBOL(rbh_insert);
+
+/**
+ * rbh_remove - Remove a pointer from a rosebush.
+ * @rbh: The rosebush.
+ * @hash: The hash value for this pointer.
+ * @p: The pointer to remove.
+ *
+ * Return: 0 on success, -ENOENT if this pointer could not be found.
+ */
+int rbh_remove(struct rbh *rbh, u32 hash, void *p)
+{
+	struct rbh_bucket *bucket;
+	unsigned int i;
+	int err = -ENOENT;
+
+	rcu_read_lock();
+	bucket = lock_bucket(rbh, hash);
+	if (!bucket)
+		goto rcu_unlock;
+
+	for (i = 0; i < RBH_ENTRIES; i++) {
+		if (bucket->rbh_hashes[i] != hash)
+			continue;
+		if (rcu_dereference_protected(bucket->rbh_slots[i],
+				lockdep_is_held(&bucket->rbh_lock)) != p)
+			continue;
+		bucket->rbh_hashes[i] = ~hash;
+		/* Do not modify the slot */
+		err = 0;
+		break;
+	}
+
+	spin_unlock(&bucket->rbh_lock);
+rcu_unlock:
+	rcu_read_unlock();
+	return err;
+}
+EXPORT_SYMBOL(rbh_remove);
+
+/**
+ * rbh_reserve - Reserve a slot in a rosebush for later use.
+ * @rbh: The rosebush.
+ * @hash: The hash value that will be used.
+ *
+ * Some callers need to take another lock before inserting an object
+ * into the rosebush.  This function reserves space for them to do that.
+ * A subsequent call to rbh_use() will not allocate memory.  If you find
+ * that you do not need the reserved space any more, call rbh_remove(),
+ * passing NULL as the pointer.
+ *
+ * Return: 0 on success, -ENOMEM on failure.
+ */
+int rbh_reserve(struct rbh *rbh, u32 hash)
+{
+	return __rbh_insert(rbh, hash, NULL);
+}
+EXPORT_SYMBOL(rbh_reserve);
+
+/**
+ * rbh_use - Use a reserved slot in a rosebush.
+ * @rbh: The rosebush.
+ * @hash: The hash value for this pointer.
+ * @p: The pointer to add.
+ *
+ * Return: 0 on success, -EINVAL if @p is NULL,
+ * -ENOENT if no reserved slot could be found.
+ */
+int rbh_use(struct rbh *rbh, u32 hash, void *p)
+{
+	struct rbh_bucket *bucket;
+	unsigned int i;
+	int err = -ENOENT;
+
+	rcu_read_lock();
+	bucket = lock_bucket(rbh, hash);
+	if (!bucket)
+		goto rcu_unlock;
+
+	for (i = 0; i < RBH_ENTRIES; i++) {
+		if (bucket->rbh_hashes[i] != hash)
+			continue;
+		if (bucket->rbh_slots[i] != NULL)
+			continue;
+		rcu_assign_pointer(bucket->rbh_slots[i], p);
+		err = 0;
+		break;
+	}
+
+	spin_unlock(&bucket->rbh_lock);
+rcu_unlock:
+	rcu_read_unlock();
+	return err;
+}
+EXPORT_SYMBOL(rbh_use);
+
+/**
+ * rbh_next - Find the next entry matching this hash
+ * @rbhi: The rosebush iterator.
+ *
+ * Return: NULL if there are no more matching hash values, otherwise
+ * the next pointer.
+ */
+void *rbh_next(struct rbh_iter *rbhi)
+{
+	struct table_mask state;
+	struct rbh_bucket *bucket = rbhi->bucket;
+	void *p;
+
+	if (!bucket) {
+		bucket = get_bucket(rbhi->rbh, rbhi->hash, &state);
+		if (!bucket)
+			return NULL;
+		rbhi->bucket = bucket;
+		rbhi->index = UINT_MAX;
+	}
+
+	while (++rbhi->index < RBH_ENTRIES) {
+		if (READ_ONCE(bucket->rbh_hashes[rbhi->index]) != rbhi->hash)
+			continue;
+		p = rcu_dereference(bucket->rbh_slots[rbhi->index]);
+		if (p)
+			return p;
+	}
+
+	return NULL;
+}
+EXPORT_SYMBOL(rbh_next);
+
+/**
+ * rbh_destroy - Destroy a rosebush
+ * @rbh: The rosebush to destroy
+ *
+ * The caller must ensure that no other threads will simultaneously
+ * attempt to use the rosebush.
+ */
+void rbh_destroy(struct rbh *rbh)
+{
+	unsigned long tagged = rbh->rbh_table;
+	struct rbh_table *table = (struct rbh_table *)(tagged & (tagged + 1));
+	u32 mask = tagged - (unsigned long)table;
+	u32 i, j, k;
+
+	for (i = 0; i <= mask; i++) {
+		struct rbh_bucket __rcu *bucket;
+
+		bucket = table->buckets[i];
+		if (!bucket)
+			continue;
+		kfree((void __force *)bucket);
+		for (j = (mask + 1) / 2; j > 0; j /= 2) {
+			if (table->buckets[i ^ j] == bucket)
+				continue;
+			j *= 2;
+			break;
+		}
+		k = i;
+		do {
+			table->buckets[k] = NULL;
+			k += j;
+		} while (k < mask && table->buckets[k] == bucket);
+	}
+
+	kfree(table);
+	rbh->rbh_table = 0;
+}
+
+#ifndef __KERNEL__
+static void dump_bucket(struct rbh_bucket *bucket, unsigned bnr)
+{
+	unsigned i;
+
+	printk("bucket:%p index:%d rbh:%p\n", bucket, bnr, bucket->rbh);
+	for (i = 0; i < RBH_ENTRIES; i++) {
+		printk("hash:%x entry:%p (%x)\n", bucket->rbh_hashes[i],
+			bucket->rbh_slots[i], bucket->rbh_hashes[i] * 2 + 1);
+	}
+}
+
+static void rbh_dump(struct rbh *rbh)
+{
+	unsigned long tagged = READ_ONCE(rbh->rbh_table);
+	struct rbh_table *table;
+	u32 mask, i;
+
+	table = (struct rbh_table *)(tagged & (tagged + 1));
+	mask = tagged - (unsigned long)table;
+
+	printk("rosebush %p has %d buckets in table %p\n", rbh, mask + 1, table);
+
+	if (mask == 1) {
+		dump_bucket((struct rbh_bucket *)table, 0);
+		dump_bucket((struct rbh_bucket *)table + 1, 1);
+	} else for (i = 0; i <= mask; i++)
+		dump_bucket(table->buckets[i], i);
+}
+#endif
+
+/*
+ * TODO:
+ * * convert the dcache
+ * * 2 byte hashes in the bucket.  Once the table has 2^17 buckets, we can
+ *   use 10 bytes per entry instead of 12 (24 entries/bucket instead of 20)
+ * * 1 byte hashes in the bucket.  Once the table has 2^25 buckets, we can
+ *   use 9 bytes per entry instead of 10 (26 entries/bucket instead of 24)
+ */
-- 
2.43.0

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