Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-nextgrafted

Pull networking updates from Jakub Kicinski:
 "Core:

   - Add dedicated kmem_cache for typical/small skb->head, avoid having
     to access struct page at kfree time, and improve memory use.

   - Introduce sysctl to set default RPS configuration for new netdevs.

   - Define Netlink protocol specification format which can be used to
     describe messages used by each family and auto-generate parsers.
     Add tools for generating kernel data structures and uAPI headers.

   - Expose all net/core sysctls inside netns.

   - Remove 4s sleep in netpoll if carrier is instantly detected on
     boot.

   - Add configurable limit of MDB entries per port, and port-vlan.

   - Continue populating drop reasons throughout the stack.

   - Retire a handful of legacy Qdiscs and classifiers.

  Protocols:

   - Support IPv4 big TCP (TSO frames larger than 64kB).

   - Add IP_LOCAL_PORT_RANGE socket option, to control local port range
     on socket by socket basis.

   - Track and report in procfs number of MPTCP sockets used.

   - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path
     manager.

   - IPv6: don't check net.ipv6.route.max_size and rely on garbage
     collection to free memory (similarly to IPv4).

   - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986).

   - ICMP: add per-rate limit counters.

   - Add support for user scanning requests in ieee802154.

   - Remove static WEP support.

   - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate
     reporting.

   - WiFi 7 EHT channel puncturing support (client & AP).

  BPF:

   - Add a rbtree data structure following the "next-gen data structure"
     precedent set by recently added linked list, that is, by using
     kfunc + kptr instead of adding a new BPF map type.

   - Expose XDP hints via kfuncs with initial support for RX hash and
     timestamp metadata.

   - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to
     better support decap on GRE tunnel devices not operating in collect
     metadata.

   - Improve x86 JIT's codegen for PROBE_MEM runtime error checks.

   - Remove the need for trace_printk_lock for bpf_trace_printk and
     bpf_trace_vprintk helpers.

   - Extend libbpf's bpf_tracing.h support for tracing arguments of
     kprobes/uprobes and syscall as a special case.

   - Significantly reduce the search time for module symbols by
     livepatch and BPF.

   - Enable cpumasks to be used as kptrs, which is useful for tracing
     programs tracking which tasks end up running on which CPUs in
     different time intervals.

   - Add support for BPF trampoline on s390x and riscv64.

   - Add capability to export the XDP features supported by the NIC.

   - Add __bpf_kfunc tag for marking kernel functions as kfuncs.

   - Add cgroup.memory=nobpf kernel parameter option to disable BPF
     memory accounting for container environments.

  Netfilter:

   - Remove the CLUSTERIP target. It has been marked as obsolete for
     years, and we still have WARN splats wrt races of the out-of-band
     /proc interface installed by this target.

   - Add 'destroy' commands to nf_tables. They are identical to the
     existing 'delete' commands, but do not return an error if the
     referenced object (set, chain, rule...) did not exist.

  Driver API:

   - Improve cpumask_local_spread() locality to help NICs set the right
     IRQ affinity on AMD platforms.

   - Separate C22 and C45 MDIO bus transactions more clearly.

   - Introduce new DCB table to control DSCP rewrite on egress.

   - Support configuration of Physical Layer Collision Avoidance (PLCA)
     Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of
     shared medium Ethernet.

   - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing
     preemption of low priority frames by high priority frames.

   - Add support for controlling MACSec offload using netlink SET.

   - Rework devlink instance refcounts to allow registration and
     de-registration under the instance lock. Split the code into
     multiple files, drop some of the unnecessarily granular locks and
     factor out common parts of netlink operation handling.

   - Add TX frame aggregation parameters (for USB drivers).

   - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning
     messages with notifications for debug.

   - Allow offloading of UDP NEW connections via act_ct.

   - Add support for per action HW stats in TC.

   - Support hardware miss to TC action (continue processing in SW from
     a specific point in the action chain).

   - Warn if old Wireless Extension user space interface is used with
     modern cfg80211/mac80211 drivers. Do not support Wireless
     Extensions for Wi-Fi 7 devices at all. Everyone should switch to
     using nl80211 interface instead.

   - Improve the CAN bit timing configuration. Use extack to return
     error messages directly to user space, update the SJW handling,
     including the definition of a new default value that will benefit
     CAN-FD controllers, by increasing their oscillator tolerance.

  New hardware / drivers:

   - Ethernet:
      - nVidia BlueField-3 support (control traffic driver)
      - Ethernet support for imx93 SoCs
      - Motorcomm yt8531 gigabit Ethernet PHY
      - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA)
      - Microchip LAN8841 PHY (incl. cable diagnostics and PTP)
      - Amlogic gxl MDIO mux

   - WiFi:
      - RealTek RTL8188EU (rtl8xxxu)
      - Qualcomm Wi-Fi 7 devices (ath12k)

   - CAN:
      - Renesas R-Car V4H

  Drivers:

   - Bluetooth:
      - Set Per Platform Antenna Gain (PPAG) for Intel controllers.

   - Ethernet NICs:
      - Intel (1G, igc):
         - support TSN / Qbv / packet scheduling features of i226 model
      - Intel (100G, ice):
         - use GNSS subsystem instead of TTY
         - multi-buffer XDP support
         - extend support for GPIO pins to E823 devices
      - nVidia/Mellanox:
         - update the shared buffer configuration on PFC commands
         - implement PTP adjphase function for HW offset control
         - TC support for Geneve and GRE with VF tunnel offload
         - more efficient crypto key management method
         - multi-port eswitch support
      - Netronome/Corigine:
         - add DCB IEEE support
         - support IPsec offloading for NFP3800
      - Freescale/NXP (enetc):
         - support XDP_REDIRECT for XDP non-linear buffers
         - improve reconfig, avoid link flap and waiting for idle
         - support MAC Merge layer
      - Other NICs:
         - sfc/ef100: add basic devlink support for ef100
         - ionic: rx_push mode operation (writing descriptors via MMIO)
         - bnxt: use the auxiliary bus abstraction for RDMA
         - r8169: disable ASPM and reset bus in case of tx timeout
         - cpsw: support QSGMII mode for J721e CPSW9G
         - cpts: support pulse-per-second output
         - ngbe: add an mdio bus driver
         - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing
         - r8152: handle devices with FW with NCM support
         - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation
         - virtio-net: support multi buffer XDP
         - virtio/vsock: replace virtio_vsock_pkt with sk_buff
         - tsnep: XDP support

   - Ethernet high-speed switches:
      - nVidia/Mellanox (mlxsw):
         - add support for latency TLV (in FW control messages)
      - Microchip (sparx5):
         - separate explicit and implicit traffic forwarding rules, make
           the implicit rules always active
         - add support for egress DSCP rewrite
         - IS0 VCAP support (Ingress Classification)
         - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS
           etc.)
         - ES2 VCAP support (Egress Access Control)
         - support for Per-Stream Filtering and Policing (802.1Q,
           8.6.5.1)

   - Ethernet embedded switches:
      - Marvell (mv88e6xxx):
         - add MAB (port auth) offload support
         - enable PTP receive for mv88e6390
      - NXP (ocelot):
         - support MAC Merge layer
         - support for the the vsc7512 internal copper phys
      - Microchip:
         - lan9303: convert to PHYLINK
         - lan966x: support TC flower filter statistics
         - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x
         - lan937x: support Credit Based Shaper configuration
         - ksz9477: support Energy Efficient Ethernet
      - other:
         - qca8k: convert to regmap read/write API, use bulk operations
         - rswitch: Improve TX timestamp accuracy

   - Intel WiFi (iwlwifi):
      - EHT (Wi-Fi 7) rate reporting
      - STEP equalizer support: transfer some STEP (connection to radio
        on platforms with integrated wifi) related parameters from the
        BIOS to the firmware.

   - Qualcomm 802.11ax WiFi (ath11k):
      - IPQ5018 support
      - Fine Timing Measurement (FTM) responder role support
      - channel 177 support

   - MediaTek WiFi (mt76):
      - per-PHY LED support
      - mt7996: EHT (Wi-Fi 7) support
      - Wireless Ethernet Dispatch (WED) reset support
      - switch to using page pool allocator

   - RealTek WiFi (rtw89):
      - support new version of Bluetooth co-existance

   - Mobile:
      - rmnet: support TX aggregation"

* tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits)
  page_pool: add a comment explaining the fragment counter usage
  net: ethtool: fix __ethtool_dev_mm_supported() implementation
  ethtool: pse-pd: Fix double word in comments
  xsk: add linux/vmalloc.h to xsk.c
  sefltests: netdevsim: wait for devlink instance after netns removal
  selftest: fib_tests: Always cleanup before exit
  net/mlx5e: Align IPsec ASO result memory to be as required by hardware
  net/mlx5e: TC, Set CT miss to the specific ct action instance
  net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG
  net/mlx5: Refactor tc miss handling to a single function
  net/mlx5: Kconfig: Make tc offload depend on tc skb extension
  net/sched: flower: Support hardware miss to tc action
  net/sched: flower: Move filter handle initialization earlier
  net/sched: cls_api: Support hardware miss to tc action
  net/sched: Rename user cookie and act cookie
  sfc: fix builds without CONFIG_RTC_LIB
  sfc: clean up some inconsistent indentings
  net/mlx4_en: Introduce flexible array to silence overflow warning
  net: lan966x: Fix possible deadlock inside PTP
  net/ulp: Remove redundant ->clone() test in inet_clone_ulp().
  ...

1 files changed, 554 insertions, 0 deletions

diff --git a/Documentation/core-api/assoc_array.rst b/Documentation/core-api/assoc_array.rst
new file mode 100644
index 000000000..792bbf993
--- /dev/null
+++ b/Documentation/core-api/assoc_array.rst
@@ -0,0 +1,554 @@
+========================================
+Generic Associative Array Implementation
+========================================
+
+Overview
+========
+
+This associative array implementation is an object container with the following
+properties:
+
+1. Objects are opaque pointers.  The implementation does not care where they
+   point (if anywhere) or what they point to (if anything).
+
+   .. note::
+
+      Pointers to objects _must_ be zero in the least significant bit.
+
+2. Objects do not need to contain linkage blocks for use by the array.  This
+   permits an object to be located in multiple arrays simultaneously.
+   Rather, the array is made up of metadata blocks that point to objects.
+
+3. Objects require index keys to locate them within the array.
+
+4. Index keys must be unique.  Inserting an object with the same key as one
+   already in the array will replace the old object.
+
+5. Index keys can be of any length and can be of different lengths.
+
+6. Index keys should encode the length early on, before any variation due to
+   length is seen.
+
+7. Index keys can include a hash to scatter objects throughout the array.
+
+8. The array can iterated over.  The objects will not necessarily come out in
+   key order.
+
+9. The array can be iterated over while it is being modified, provided the
+   RCU readlock is being held by the iterator.  Note, however, under these
+   circumstances, some objects may be seen more than once.  If this is a
+   problem, the iterator should lock against modification.  Objects will not
+   be missed, however, unless deleted.
+
+10. Objects in the array can be looked up by means of their index key.
+
+11. Objects can be looked up while the array is being modified, provided the
+    RCU readlock is being held by the thread doing the look up.
+
+The implementation uses a tree of 16-pointer nodes internally that are indexed
+on each level by nibbles from the index key in the same manner as in a radix
+tree.  To improve memory efficiency, shortcuts can be emplaced to skip over
+what would otherwise be a series of single-occupancy nodes.  Further, nodes
+pack leaf object pointers into spare space in the node rather than making an
+extra branch until as such time an object needs to be added to a full node.
+
+
+The Public API
+==============
+
+The public API can be found in ``<linux/assoc_array.h>``.  The associative
+array is rooted on the following structure::
+
+    struct assoc_array {
+            ...
+    };
+
+The code is selected by enabling ``CONFIG_ASSOCIATIVE_ARRAY`` with::
+
+    ./script/config -e ASSOCIATIVE_ARRAY
+
+
+Edit Script
+-----------
+
+The insertion and deletion functions produce an 'edit script' that can later be
+applied to effect the changes without risking ``ENOMEM``. This retains the
+preallocated metadata blocks that will be installed in the internal tree and
+keeps track of the metadata blocks that will be removed from the tree when the
+script is applied.
+
+This is also used to keep track of dead blocks and dead objects after the
+script has been applied so that they can be freed later.  The freeing is done
+after an RCU grace period has passed - thus allowing access functions to
+proceed under the RCU read lock.
+
+The script appears as outside of the API as a pointer of the type::
+
+    struct assoc_array_edit;
+
+There are two functions for dealing with the script:
+
+1. Apply an edit script::
+
+    void assoc_array_apply_edit(struct assoc_array_edit *edit);
+
+This will perform the edit functions, interpolating various write barriers
+to permit accesses under the RCU read lock to continue.  The edit script
+will then be passed to ``call_rcu()`` to free it and any dead stuff it points
+to.
+
+2. Cancel an edit script::
+
+    void assoc_array_cancel_edit(struct assoc_array_edit *edit);
+
+This frees the edit script and all preallocated memory immediately. If
+this was for insertion, the new object is _not_ released by this function,
+but must rather be released by the caller.
+
+These functions are guaranteed not to fail.
+
+
+Operations Table
+----------------
+
+Various functions take a table of operations::
+
+    struct assoc_array_ops {
+            ...
+    };
+
+This points to a number of methods, all of which need to be provided:
+
+1. Get a chunk of index key from caller data::
+
+    unsigned long (*get_key_chunk)(const void *index_key, int level);
+
+This should return a chunk of caller-supplied index key starting at the
+*bit* position given by the level argument.  The level argument will be a
+multiple of ``ASSOC_ARRAY_KEY_CHUNK_SIZE`` and the function should return
+``ASSOC_ARRAY_KEY_CHUNK_SIZE bits``.  No error is possible.
+
+
+2. Get a chunk of an object's index key::
+
+    unsigned long (*get_object_key_chunk)(const void *object, int level);
+
+As the previous function, but gets its data from an object in the array
+rather than from a caller-supplied index key.
+
+
+3. See if this is the object we're looking for::
+
+    bool (*compare_object)(const void *object, const void *index_key);
+
+Compare the object against an index key and return ``true`` if it matches and
+``false`` if it doesn't.
+
+
+4. Diff the index keys of two objects::
+
+    int (*diff_objects)(const void *object, const void *index_key);
+
+Return the bit position at which the index key of the specified object
+differs from the given index key or -1 if they are the same.
+
+
+5. Free an object::
+
+    void (*free_object)(void *object);
+
+Free the specified object.  Note that this may be called an RCU grace period
+after ``assoc_array_apply_edit()`` was called, so ``synchronize_rcu()`` may be
+necessary on module unloading.
+
+
+Manipulation Functions
+----------------------
+
+There are a number of functions for manipulating an associative array:
+
+1. Initialise an associative array::
+
+    void assoc_array_init(struct assoc_array *array);
+
+This initialises the base structure for an associative array.  It can't fail.
+
+
+2. Insert/replace an object in an associative array::
+
+    struct assoc_array_edit *
+    assoc_array_insert(struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       const void *index_key,
+                       void *object);
+
+This inserts the given object into the array.  Note that the least
+significant bit of the pointer must be zero as it's used to type-mark
+pointers internally.
+
+If an object already exists for that key then it will be replaced with the
+new object and the old one will be freed automatically.
+
+The ``index_key`` argument should hold index key information and is
+passed to the methods in the ops table when they are called.
+
+This function makes no alteration to the array itself, but rather returns
+an edit script that must be applied.  ``-ENOMEM`` is returned in the case of
+an out-of-memory error.
+
+The caller should lock exclusively against other modifiers of the array.
+
+
+3. Delete an object from an associative array::
+
+    struct assoc_array_edit *
+    assoc_array_delete(struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       const void *index_key);
+
+This deletes an object that matches the specified data from the array.
+
+The ``index_key`` argument should hold index key information and is
+passed to the methods in the ops table when they are called.
+
+This function makes no alteration to the array itself, but rather returns
+an edit script that must be applied.  ``-ENOMEM`` is returned in the case of
+an out-of-memory error.  ``NULL`` will be returned if the specified object is
+not found within the array.
+
+The caller should lock exclusively against other modifiers of the array.
+
+
+4. Delete all objects from an associative array::
+
+    struct assoc_array_edit *
+    assoc_array_clear(struct assoc_array *array,
+                      const struct assoc_array_ops *ops);
+
+This deletes all the objects from an associative array and leaves it
+completely empty.
+
+This function makes no alteration to the array itself, but rather returns
+an edit script that must be applied.  ``-ENOMEM`` is returned in the case of
+an out-of-memory error.
+
+The caller should lock exclusively against other modifiers of the array.
+
+
+5. Destroy an associative array, deleting all objects::
+
+    void assoc_array_destroy(struct assoc_array *array,
+                             const struct assoc_array_ops *ops);
+
+This destroys the contents of the associative array and leaves it
+completely empty.  It is not permitted for another thread to be traversing
+the array under the RCU read lock at the same time as this function is
+destroying it as no RCU deferral is performed on memory release -
+something that would require memory to be allocated.
+
+The caller should lock exclusively against other modifiers and accessors
+of the array.
+
+
+6. Garbage collect an associative array::
+
+    int assoc_array_gc(struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       bool (*iterator)(void *object, void *iterator_data),
+                       void *iterator_data);
+
+This iterates over the objects in an associative array and passes each one to
+``iterator()``.  If ``iterator()`` returns ``true``, the object is kept.  If it
+returns ``false``, the object will be freed.  If the ``iterator()`` function
+returns ``true``, it must perform any appropriate refcount incrementing on the
+object before returning.
+
+The internal tree will be packed down if possible as part of the iteration
+to reduce the number of nodes in it.
+
+The ``iterator_data`` is passed directly to ``iterator()`` and is otherwise
+ignored by the function.
+
+The function will return ``0`` if successful and ``-ENOMEM`` if there wasn't
+enough memory.
+
+It is possible for other threads to iterate over or search the array under
+the RCU read lock while this function is in progress.  The caller should
+lock exclusively against other modifiers of the array.
+
+
+Access Functions
+----------------
+
+There are two functions for accessing an associative array:
+
+1. Iterate over all the objects in an associative array::
+
+    int assoc_array_iterate(const struct assoc_array *array,
+                            int (*iterator)(const void *object,
+                                            void *iterator_data),
+                            void *iterator_data);
+
+This passes each object in the array to the iterator callback function.
+``iterator_data`` is private data for that function.
+
+This may be used on an array at the same time as the array is being
+modified, provided the RCU read lock is held.  Under such circumstances,
+it is possible for the iteration function to see some objects twice.  If
+this is a problem, then modification should be locked against.  The
+iteration algorithm should not, however, miss any objects.
+
+The function will return ``0`` if no objects were in the array or else it will
+return the result of the last iterator function called.  Iteration stops
+immediately if any call to the iteration function results in a non-zero
+return.
+
+
+2. Find an object in an associative array::
+
+    void *assoc_array_find(const struct assoc_array *array,
+                           const struct assoc_array_ops *ops,
+                           const void *index_key);
+
+This walks through the array's internal tree directly to the object
+specified by the index key..
+
+This may be used on an array at the same time as the array is being
+modified, provided the RCU read lock is held.
+
+The function will return the object if found (and set ``*_type`` to the object
+type) or will return ``NULL`` if the object was not found.
+
+
+Index Key Form
+--------------
+
+The index key can be of any form, but since the algorithms aren't told how long
+the key is, it is strongly recommended that the index key includes its length
+very early on before any variation due to the length would have an effect on
+comparisons.
+
+This will cause leaves with different length keys to scatter away from each
+other - and those with the same length keys to cluster together.
+
+It is also recommended that the index key begin with a hash of the rest of the
+key to maximise scattering throughout keyspace.
+
+The better the scattering, the wider and lower the internal tree will be.
+
+Poor scattering isn't too much of a problem as there are shortcuts and nodes
+can contain mixtures of leaves and metadata pointers.
+
+The index key is read in chunks of machine word.  Each chunk is subdivided into
+one nibble (4 bits) per level, so on a 32-bit CPU this is good for 8 levels and
+on a 64-bit CPU, 16 levels.  Unless the scattering is really poor, it is
+unlikely that more than one word of any particular index key will have to be
+used.
+
+
+Internal Workings
+=================
+
+The associative array data structure has an internal tree.  This tree is
+constructed of two types of metadata blocks: nodes and shortcuts.
+
+A node is an array of slots.  Each slot can contain one of four things:
+
+* A NULL pointer, indicating that the slot is empty.
+* A pointer to an object (a leaf).
+* A pointer to a node at the next level.
+* A pointer to a shortcut.
+
+
+Basic Internal Tree Layout
+--------------------------
+
+Ignoring shortcuts for the moment, the nodes form a multilevel tree.  The index
+key space is strictly subdivided by the nodes in the tree and nodes occur on
+fixed levels.  For example::
+
+ Level: 0               1               2               3
+        =============== =============== =============== ===============
+                                                        NODE D
+                        NODE B          NODE C  +------>+---+
+                +------>+---+   +------>+---+   |       | 0 |
+        NODE A  |       | 0 |   |       | 0 |   |       +---+
+        +---+   |       +---+   |       +---+   |       :   :
+        | 0 |   |       :   :   |       :   :   |       +---+
+        +---+   |       +---+   |       +---+   |       | f |
+        | 1 |---+       | 3 |---+       | 7 |---+       +---+
+        +---+           +---+           +---+
+        :   :           :   :           | 8 |---+
+        +---+           +---+           +---+   |       NODE E
+        | e |---+       | f |           :   :   +------>+---+
+        +---+   |       +---+           +---+           | 0 |
+        | f |   |                       | f |           +---+
+        +---+   |                       +---+           :   :
+                |       NODE F                          +---+
+                +------>+---+                           | f |
+                        | 0 |           NODE G          +---+
+                        +---+   +------>+---+
+                        :   :   |       | 0 |
+                        +---+   |       +---+
+                        | 6 |---+       :   :
+                        +---+           +---+
+                        :   :           | f |
+                        +---+           +---+
+                        | f |
+                        +---+
+
+In the above example, there are 7 nodes (A-G), each with 16 slots (0-f).
+Assuming no other meta data nodes in the tree, the key space is divided
+thusly::
+
+    KEY PREFIX      NODE
+    ==========      ====
+    137*            D
+    138*            E
+    13[0-69-f]*     C
+    1[0-24-f]*      B
+    e6*             G
+    e[0-57-f]*      F
+    [02-df]*        A
+
+So, for instance, keys with the following example index keys will be found in
+the appropriate nodes::
+
+    INDEX KEY       PREFIX  NODE
+    =============== ======= ====
+    13694892892489  13      C
+    13795289025897  137     D
+    13889dde88793   138     E
+    138bbb89003093  138     E
+    1394879524789   12      C
+    1458952489      1       B
+    9431809de993ba  -       A
+    b4542910809cd   -       A
+    e5284310def98   e       F
+    e68428974237    e6      G
+    e7fffcbd443     e       F
+    f3842239082     -       A
+
+To save memory, if a node can hold all the leaves in its portion of keyspace,
+then the node will have all those leaves in it and will not have any metadata
+pointers - even if some of those leaves would like to be in the same slot.
+
+A node can contain a heterogeneous mix of leaves and metadata pointers.
+Metadata pointers must be in the slots that match their subdivisions of key
+space.  The leaves can be in any slot not occupied by a metadata pointer.  It
+is guaranteed that none of the leaves in a node will match a slot occupied by a
+metadata pointer.  If the metadata pointer is there, any leaf whose key matches
+the metadata key prefix must be in the subtree that the metadata pointer points
+to.
+
+In the above example list of index keys, node A will contain::
+
+    SLOT    CONTENT         INDEX KEY (PREFIX)
+    ====    =============== ==================
+    1       PTR TO NODE B   1*
+    any     LEAF            9431809de993ba
+    any     LEAF            b4542910809cd
+    e       PTR TO NODE F   e*
+    any     LEAF            f3842239082
+
+and node B::
+
+    3	PTR TO NODE C	13*
+    any	LEAF		1458952489
+
+
+Shortcuts
+---------
+
+Shortcuts are metadata records that jump over a piece of keyspace.  A shortcut
+is a replacement for a series of single-occupancy nodes ascending through the
+levels.  Shortcuts exist to save memory and to speed up traversal.
+
+It is possible for the root of the tree to be a shortcut - say, for example,
+the tree contains at least 17 nodes all with key prefix ``1111``.  The
+insertion algorithm will insert a shortcut to skip over the ``1111`` keyspace
+in a single bound and get to the fourth level where these actually become
+different.
+
+
+Splitting And Collapsing Nodes
+------------------------------
+
+Each node has a maximum capacity of 16 leaves and metadata pointers.  If the
+insertion algorithm finds that it is trying to insert a 17th object into a
+node, that node will be split such that at least two leaves that have a common
+key segment at that level end up in a separate node rooted on that slot for
+that common key segment.
+
+If the leaves in a full node and the leaf that is being inserted are
+sufficiently similar, then a shortcut will be inserted into the tree.
+
+When the number of objects in the subtree rooted at a node falls to 16 or
+fewer, then the subtree will be collapsed down to a single node - and this will
+ripple towards the root if possible.
+
+
+Non-Recursive Iteration
+-----------------------
+
+Each node and shortcut contains a back pointer to its parent and the number of
+slot in that parent that points to it.  None-recursive iteration uses these to
+proceed rootwards through the tree, going to the parent node, slot N + 1 to
+make sure progress is made without the need for a stack.
+
+The backpointers, however, make simultaneous alteration and iteration tricky.
+
+
+Simultaneous Alteration And Iteration
+-------------------------------------
+
+There are a number of cases to consider:
+
+1. Simple insert/replace.  This involves simply replacing a NULL or old
+   matching leaf pointer with the pointer to the new leaf after a barrier.
+   The metadata blocks don't change otherwise.  An old leaf won't be freed
+   until after the RCU grace period.
+
+2. Simple delete.  This involves just clearing an old matching leaf.  The
+   metadata blocks don't change otherwise.  The old leaf won't be freed until
+   after the RCU grace period.
+
+3. Insertion replacing part of a subtree that we haven't yet entered.  This
+   may involve replacement of part of that subtree - but that won't affect
+   the iteration as we won't have reached the pointer to it yet and the
+   ancestry blocks are not replaced (the layout of those does not change).
+
+4. Insertion replacing nodes that we're actively processing.  This isn't a
+   problem as we've passed the anchoring pointer and won't switch onto the
+   new layout until we follow the back pointers - at which point we've
+   already examined the leaves in the replaced node (we iterate over all the
+   leaves in a node before following any of its metadata pointers).
+
+   We might, however, re-see some leaves that have been split out into a new
+   branch that's in a slot further along than we were at.
+
+5. Insertion replacing nodes that we're processing a dependent branch of.
+   This won't affect us until we follow the back pointers.  Similar to (4).
+
+6. Deletion collapsing a branch under us.  This doesn't affect us because the
+   back pointers will get us back to the parent of the new node before we
+   could see the new node.  The entire collapsed subtree is thrown away
+   unchanged - and will still be rooted on the same slot, so we shouldn't
+   process it a second time as we'll go back to slot + 1.
+
+.. note::
+
+   Under some circumstances, we need to simultaneously change the parent
+   pointer and the parent slot pointer on a node (say, for example, we
+   inserted another node before it and moved it up a level).  We cannot do
+   this without locking against a read - so we have to replace that node too.
+
+   However, when we're changing a shortcut into a node this isn't a problem
+   as shortcuts only have one slot and so the parent slot number isn't used
+   when traversing backwards over one.  This means that it's okay to change
+   the slot number first - provided suitable barriers are used to make sure
+   the parent slot number is read after the back pointer.
+
+Obsolete blocks and leaves are freed up after an RCU grace period has passed,
+so as long as anyone doing walking or iteration holds the RCU read lock, the
+old superstructure should not go away on them.