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, 983 insertions, 0 deletions

diff --git a/Documentation/trace/ring-buffer-design.rst b/Documentation/trace/ring-buffer-design.rst
new file mode 100644
index 000000000..c5d77fcbb
--- /dev/null
+++ b/Documentation/trace/ring-buffer-design.rst
@@ -0,0 +1,983 @@
+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.2-no-invariants-only
+
+===========================
+Lockless Ring Buffer Design
+===========================
+
+Copyright 2009 Red Hat Inc.
+
+:Author:   Steven Rostedt <srostedt@redhat.com>
+:License:  The GNU Free Documentation License, Version 1.2
+           (dual licensed under the GPL v2)
+:Reviewers:  Mathieu Desnoyers, Huang Ying, Hidetoshi Seto,
+	     and Frederic Weisbecker.
+
+
+Written for: 2.6.31
+
+Terminology used in this Document
+---------------------------------
+
+tail
+	- where new writes happen in the ring buffer.
+
+head
+	- where new reads happen in the ring buffer.
+
+producer
+	- the task that writes into the ring buffer (same as writer)
+
+writer
+	- same as producer
+
+consumer
+	- the task that reads from the buffer (same as reader)
+
+reader
+	- same as consumer.
+
+reader_page
+	- A page outside the ring buffer used solely (for the most part)
+	  by the reader.
+
+head_page
+	- a pointer to the page that the reader will use next
+
+tail_page
+	- a pointer to the page that will be written to next
+
+commit_page
+	- a pointer to the page with the last finished non-nested write.
+
+cmpxchg
+	- hardware-assisted atomic transaction that performs the following::
+
+	    A = B if previous A == C
+
+	    R = cmpxchg(A, C, B) is saying that we replace A with B if and only
+		if current A is equal to C, and we put the old (current)
+		A into R
+
+	    R gets the previous A regardless if A is updated with B or not.
+
+	  To see if the update was successful a compare of ``R == C``
+	  may be used.
+
+The Generic Ring Buffer
+-----------------------
+
+The ring buffer can be used in either an overwrite mode or in
+producer/consumer mode.
+
+Producer/consumer mode is where if the producer were to fill up the
+buffer before the consumer could free up anything, the producer
+will stop writing to the buffer. This will lose most recent events.
+
+Overwrite mode is where if the producer were to fill up the buffer
+before the consumer could free up anything, the producer will
+overwrite the older data. This will lose the oldest events.
+
+No two writers can write at the same time (on the same per-cpu buffer),
+but a writer may interrupt another writer, but it must finish writing
+before the previous writer may continue. This is very important to the
+algorithm. The writers act like a "stack". The way interrupts works
+enforces this behavior::
+
+
+  writer1 start
+     <preempted> writer2 start
+         <preempted> writer3 start
+                     writer3 finishes
+                 writer2 finishes
+  writer1 finishes
+
+This is very much like a writer being preempted by an interrupt and
+the interrupt doing a write as well.
+
+Readers can happen at any time. But no two readers may run at the
+same time, nor can a reader preempt/interrupt another reader. A reader
+cannot preempt/interrupt a writer, but it may read/consume from the
+buffer at the same time as a writer is writing, but the reader must be
+on another processor to do so. A reader may read on its own processor
+and can be preempted by a writer.
+
+A writer can preempt a reader, but a reader cannot preempt a writer.
+But a reader can read the buffer at the same time (on another processor)
+as a writer.
+
+The ring buffer is made up of a list of pages held together by a linked list.
+
+At initialization a reader page is allocated for the reader that is not
+part of the ring buffer.
+
+The head_page, tail_page and commit_page are all initialized to point
+to the same page.
+
+The reader page is initialized to have its next pointer pointing to
+the head page, and its previous pointer pointing to a page before
+the head page.
+
+The reader has its own page to use. At start up time, this page is
+allocated but is not attached to the list. When the reader wants
+to read from the buffer, if its page is empty (like it is on start-up),
+it will swap its page with the head_page. The old reader page will
+become part of the ring buffer and the head_page will be removed.
+The page after the inserted page (old reader_page) will become the
+new head page.
+
+Once the new page is given to the reader, the reader could do what
+it wants with it, as long as a writer has left that page.
+
+A sample of how the reader page is swapped: Note this does not
+show the head page in the buffer, it is for demonstrating a swap
+only.
+
+::
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |
+  +------+
+                  +---+   +---+   +---+
+                  |   |-->|   |-->|   |
+                  |   |<--|   |<--|   |
+                  +---+   +---+   +---+
+                   ^ |             ^ |
+                   | +-------------+ |
+                   +-----------------+
+
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |-------------------+
+  +------+                   v
+    |             +---+   +---+   +---+
+    |             |   |-->|   |-->|   |
+    |             |   |<--|   |<--|   |<-+
+    |             +---+   +---+   +---+  |
+    |              ^ |             ^ |   |
+    |              | +-------------+ |   |
+    |              +-----------------+   |
+    +------------------------------------+
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |-------------------+
+  +------+ <---------------+ v
+    |  ^          +---+   +---+   +---+
+    |  |          |   |-->|   |-->|   |
+    |  |          |   |   |   |<--|   |<-+
+    |  |          +---+   +---+   +---+  |
+    |  |             |             ^ |   |
+    |  |             +-------------+ |   |
+    |  +-----------------------------+   |
+    +------------------------------------+
+
+  +------+
+  |buffer|          RING BUFFER
+  |page  |-------------------+
+  +------+ <---------------+ v
+    |  ^          +---+   +---+   +---+
+    |  |          |   |   |   |-->|   |
+    |  |  New     |   |   |   |<--|   |<-+
+    |  | Reader   +---+   +---+   +---+  |
+    |  |  page ----^                 |   |
+    |  |                             |   |
+    |  +-----------------------------+   |
+    +------------------------------------+
+
+
+
+It is possible that the page swapped is the commit page and the tail page,
+if what is in the ring buffer is less than what is held in a buffer page.
+
+::
+
+            reader page    commit page   tail page
+                |              |             |
+                v              |             |
+               +---+           |             |
+               |   |<----------+             |
+               |   |<------------------------+
+               |   |------+
+               +---+      |
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+This case is still valid for this algorithm.
+When the writer leaves the page, it simply goes into the ring buffer
+since the reader page still points to the next location in the ring
+buffer.
+
+
+The main pointers:
+
+  reader page
+	    - The page used solely by the reader and is not part
+              of the ring buffer (may be swapped in)
+
+  head page
+	    - the next page in the ring buffer that will be swapped
+              with the reader page.
+
+  tail page
+	    - the page where the next write will take place.
+
+  commit page
+	    - the page that last finished a write.
+
+The commit page only is updated by the outermost writer in the
+writer stack. A writer that preempts another writer will not move the
+commit page.
+
+When data is written into the ring buffer, a position is reserved
+in the ring buffer and passed back to the writer. When the writer
+is finished writing data into that position, it commits the write.
+
+Another write (or a read) may take place at anytime during this
+transaction. If another write happens it must finish before continuing
+with the previous write.
+
+
+   Write reserve::
+
+       Buffer page
+      +---------+
+      |written  |
+      +---------+  <--- given back to writer (current commit)
+      |reserved |
+      +---------+ <--- tail pointer
+      | empty   |
+      +---------+
+
+   Write commit::
+
+       Buffer page
+      +---------+
+      |written  |
+      +---------+
+      |written  |
+      +---------+  <--- next position for write (current commit)
+      | empty   |
+      +---------+
+
+
+ If a write happens after the first reserve::
+
+       Buffer page
+      +---------+
+      |written  |
+      +---------+  <-- current commit
+      |reserved |
+      +---------+  <--- given back to second writer
+      |reserved |
+      +---------+ <--- tail pointer
+
+  After second writer commits::
+
+
+       Buffer page
+      +---------+
+      |written  |
+      +---------+  <--(last full commit)
+      |reserved |
+      +---------+
+      |pending  |
+      |commit   |
+      +---------+ <--- tail pointer
+
+  When the first writer commits::
+
+       Buffer page
+      +---------+
+      |written  |
+      +---------+
+      |written  |
+      +---------+
+      |written  |
+      +---------+  <--(last full commit and tail pointer)
+
+
+The commit pointer points to the last write location that was
+committed without preempting another write. When a write that
+preempted another write is committed, it only becomes a pending commit
+and will not be a full commit until all writes have been committed.
+
+The commit page points to the page that has the last full commit.
+The tail page points to the page with the last write (before
+committing).
+
+The tail page is always equal to or after the commit page. It may
+be several pages ahead. If the tail page catches up to the commit
+page then no more writes may take place (regardless of the mode
+of the ring buffer: overwrite and produce/consumer).
+
+The order of pages is::
+
+ head page
+ commit page
+ tail page
+
+Possible scenario::
+
+                               tail page
+    head page         commit page  |
+        |                 |        |
+        v                 v        v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+There is a special case that the head page is after either the commit page
+and possibly the tail page. That is when the commit (and tail) page has been
+swapped with the reader page. This is because the head page is always
+part of the ring buffer, but the reader page is not. Whenever there
+has been less than a full page that has been committed inside the ring buffer,
+and a reader swaps out a page, it will be swapping out the commit page.
+
+::
+
+            reader page    commit page   tail page
+                |              |             |
+                v              |             |
+               +---+           |             |
+               |   |<----------+             |
+               |   |<------------------------+
+               |   |------+
+               +---+      |
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+                          ^
+                          |
+                      head page
+
+
+In this case, the head page will not move when the tail and commit
+move back into the ring buffer.
+
+The reader cannot swap a page into the ring buffer if the commit page
+is still on that page. If the read meets the last commit (real commit
+not pending or reserved), then there is nothing more to read.
+The buffer is considered empty until another full commit finishes.
+
+When the tail meets the head page, if the buffer is in overwrite mode,
+the head page will be pushed ahead one. If the buffer is in producer/consumer
+mode, the write will fail.
+
+Overwrite mode::
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+                          ^
+                          |
+                      head page
+
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+                                   ^
+                                   |
+                               head page
+
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+                                   ^
+                                   |
+                               head page
+
+Note, the reader page will still point to the previous head page.
+But when a swap takes place, it will use the most recent head page.
+
+
+Making the Ring Buffer Lockless:
+--------------------------------
+
+The main idea behind the lockless algorithm is to combine the moving
+of the head_page pointer with the swapping of pages with the reader.
+State flags are placed inside the pointer to the page. To do this,
+each page must be aligned in memory by 4 bytes. This will allow the 2
+least significant bits of the address to be used as flags, since
+they will always be zero for the address. To get the address,
+simply mask out the flags::
+
+  MASK = ~3
+
+  address & MASK
+
+Two flags will be kept by these two bits:
+
+   HEADER
+	- the page being pointed to is a head page
+
+   UPDATE
+	- the page being pointed to is being updated by a writer
+          and was or is about to be a head page.
+
+::
+
+	      reader page
+		  |
+		  v
+		+---+
+		|   |------+
+		+---+      |
+			    |
+			    v
+	+---+    +---+    +---+    +---+
+    <---|   |--->|   |-H->|   |--->|   |--->
+    --->|   |<---|   |<---|   |<---|   |<---
+	+---+    +---+    +---+    +---+
+
+
+The above pointer "-H->" would have the HEADER flag set. That is
+the next page is the next page to be swapped out by the reader.
+This pointer means the next page is the head page.
+
+When the tail page meets the head pointer, it will use cmpxchg to
+change the pointer to the UPDATE state::
+
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-H->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+"-U->" represents a pointer in the UPDATE state.
+
+Any access to the reader will need to take some sort of lock to serialize
+the readers. But the writers will never take a lock to write to the
+ring buffer. This means we only need to worry about a single reader,
+and writes only preempt in "stack" formation.
+
+When the reader tries to swap the page with the ring buffer, it
+will also use cmpxchg. If the flag bit in the pointer to the
+head page does not have the HEADER flag set, the compare will fail
+and the reader will need to look for the new head page and try again.
+Note, the flags UPDATE and HEADER are never set at the same time.
+
+The reader swaps the reader page as follows::
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |
+  +------+
+                  +---+    +---+    +---+
+                  |   |--->|   |--->|   |
+                  |   |<---|   |<---|   |
+                  +---+    +---+    +---+
+                   ^ |               ^ |
+                   | +---------------+ |
+                   +-----H-------------+
+
+The reader sets the reader page next pointer as HEADER to the page after
+the head page::
+
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |-------H-----------+
+  +------+                   v
+    |             +---+    +---+    +---+
+    |             |   |--->|   |--->|   |
+    |             |   |<---|   |<---|   |<-+
+    |             +---+    +---+    +---+  |
+    |              ^ |               ^ |   |
+    |              | +---------------+ |   |
+    |              +-----H-------------+   |
+    +--------------------------------------+
+
+It does a cmpxchg with the pointer to the previous head page to make it
+point to the reader page. Note that the new pointer does not have the HEADER
+flag set.  This action atomically moves the head page forward::
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |-------H-----------+
+  +------+                   v
+    |  ^          +---+   +---+   +---+
+    |  |          |   |-->|   |-->|   |
+    |  |          |   |<--|   |<--|   |<-+
+    |  |          +---+   +---+   +---+  |
+    |  |             |             ^ |   |
+    |  |             +-------------+ |   |
+    |  +-----------------------------+   |
+    +------------------------------------+
+
+After the new head page is set, the previous pointer of the head page is
+updated to the reader page::
+
+  +------+
+  |reader|          RING BUFFER
+  |page  |-------H-----------+
+  +------+ <---------------+ v
+    |  ^          +---+   +---+   +---+
+    |  |          |   |-->|   |-->|   |
+    |  |          |   |   |   |<--|   |<-+
+    |  |          +---+   +---+   +---+  |
+    |  |             |             ^ |   |
+    |  |             +-------------+ |   |
+    |  +-----------------------------+   |
+    +------------------------------------+
+
+  +------+
+  |buffer|          RING BUFFER
+  |page  |-------H-----------+  <--- New head page
+  +------+ <---------------+ v
+    |  ^          +---+   +---+   +---+
+    |  |          |   |   |   |-->|   |
+    |  |  New     |   |   |   |<--|   |<-+
+    |  | Reader   +---+   +---+   +---+  |
+    |  |  page ----^                 |   |
+    |  |                             |   |
+    |  +-----------------------------+   |
+    +------------------------------------+
+
+Another important point: The page that the reader page points back to
+by its previous pointer (the one that now points to the new head page)
+never points back to the reader page. That is because the reader page is
+not part of the ring buffer. Traversing the ring buffer via the next pointers
+will always stay in the ring buffer. Traversing the ring buffer via the
+prev pointers may not.
+
+Note, the way to determine a reader page is simply by examining the previous
+pointer of the page. If the next pointer of the previous page does not
+point back to the original page, then the original page is a reader page::
+
+
+             +--------+
+             | reader |  next   +----+
+             |  page  |-------->|    |<====== (buffer page)
+             +--------+         +----+
+                 |                | ^
+                 |                v | next
+            prev |              +----+
+                 +------------->|    |
+                                +----+
+
+The way the head page moves forward:
+
+When the tail page meets the head page and the buffer is in overwrite mode
+and more writes take place, the head page must be moved forward before the
+writer may move the tail page. The way this is done is that the writer
+performs a cmpxchg to convert the pointer to the head page from the HEADER
+flag to have the UPDATE flag set. Once this is done, the reader will
+not be able to swap the head page from the buffer, nor will it be able to
+move the head page, until the writer is finished with the move.
+
+This eliminates any races that the reader can have on the writer. The reader
+must spin, and this is why the reader cannot preempt the writer::
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-H->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The following page will be made into the new head page::
+
+             tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+After the new head page has been set, we can set the old head page
+pointer back to NORMAL::
+
+             tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+After the head page has been moved, the tail page may now move forward::
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+
+The above are the trivial updates. Now for the more complex scenarios.
+
+
+As stated before, if enough writes preempt the first write, the
+tail page may make it all the way around the buffer and meet the commit
+page. At this time, we must start dropping writes (usually with some kind
+of warning to the user). But what happens if the commit was still on the
+reader page? The commit page is not part of the ring buffer. The tail page
+must account for this::
+
+
+            reader page    commit page
+                |              |
+                v              |
+               +---+           |
+               |   |<----------+
+               |   |
+               |   |------+
+               +---+      |
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-H->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+                 ^
+                 |
+             tail page
+
+If the tail page were to simply push the head page forward, the commit when
+leaving the reader page would not be pointing to the correct page.
+
+The solution to this is to test if the commit page is on the reader page
+before pushing the head page. If it is, then it can be assumed that the
+tail page wrapped the buffer, and we must drop new writes.
+
+This is not a race condition, because the commit page can only be moved
+by the outermost writer (the writer that was preempted).
+This means that the commit will not move while a writer is moving the
+tail page. The reader cannot swap the reader page if it is also being
+used as the commit page. The reader can simply check that the commit
+is off the reader page. Once the commit page leaves the reader page
+it will never go back on it unless a reader does another swap with the
+buffer page that is also the commit page.
+
+
+Nested writes
+-------------
+
+In the pushing forward of the tail page we must first push forward
+the head page if the head page is the next page. If the head page
+is not the next page, the tail page is simply updated with a cmpxchg.
+
+Only writers move the tail page. This must be done atomically to protect
+against nested writers::
+
+  temp_page = tail_page
+  next_page = temp_page->next
+  cmpxchg(tail_page, temp_page, next_page)
+
+The above will update the tail page if it is still pointing to the expected
+page. If this fails, a nested write pushed it forward, the current write
+does not need to push it::
+
+
+             temp page
+                 |
+                 v
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+Nested write comes in and moves the tail page forward::
+
+                      tail page (moved by nested writer)
+              temp page   |
+                 |        |
+                 v        v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The above would fail the cmpxchg, but since the tail page has already
+been moved forward, the writer will just try again to reserve storage
+on the new tail page.
+
+But the moving of the head page is a bit more complex::
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-H->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The write converts the head page pointer to UPDATE::
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+But if a nested writer preempts here, it will see that the next
+page is a head page, but it is also nested. It will detect that
+it is nested and will save that information. The detection is the
+fact that it sees the UPDATE flag instead of a HEADER or NORMAL
+pointer.
+
+The nested writer will set the new head page pointer::
+
+             tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+But it will not reset the update back to normal. Only the writer
+that converted a pointer from HEAD to UPDATE will convert it back
+to NORMAL::
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+After the nested writer finishes, the outermost writer will convert
+the UPDATE pointer to NORMAL::
+
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+
+It can be even more complex if several nested writes came in and moved
+the tail page ahead several pages::
+
+
+  (first writer)
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-H->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The write converts the head page pointer to UPDATE::
+
+              tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+Next writer comes in, and sees the update and sets up the new
+head page::
+
+  (second writer)
+
+             tail page
+                 |
+                 v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The nested writer moves the tail page forward. But does not set the old
+update page to NORMAL because it is not the outermost writer::
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+Another writer preempts and sees the page after the tail page is a head page.
+It changes it from HEAD to UPDATE::
+
+  (third writer)
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-U->|   |--->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The writer will move the head page forward::
+
+
+  (third writer)
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-U->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+But now that the third writer did change the HEAD flag to UPDATE it
+will convert it to normal::
+
+
+  (third writer)
+
+                      tail page
+                          |
+                          v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+
+Then it will move the tail page, and return back to the second writer::
+
+
+  (second writer)
+
+                               tail page
+                                   |
+                                   v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+
+The second writer will fail to move the tail page because it was already
+moved, so it will try again and add its data to the new tail page.
+It will return to the first writer::
+
+
+  (first writer)
+
+                               tail page
+                                   |
+                                   v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+The first writer cannot know atomically if the tail page moved
+while it updates the HEAD page. It will then update the head page to
+what it thinks is the new head page::
+
+
+  (first writer)
+
+                               tail page
+                                   |
+                                   v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+Since the cmpxchg returns the old value of the pointer the first writer
+will see it succeeded in updating the pointer from NORMAL to HEAD.
+But as we can see, this is not good enough. It must also check to see
+if the tail page is either where it use to be or on the next page::
+
+
+  (first writer)
+
+                 A        B    tail page
+                 |        |        |
+                 v        v        v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |-H->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+If tail page != A and tail page != B, then it must reset the pointer
+back to NORMAL. The fact that it only needs to worry about nested
+writers means that it only needs to check this after setting the HEAD page::
+
+
+  (first writer)
+
+                 A        B    tail page
+                 |        |        |
+                 v        v        v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |-U->|   |--->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+
+
+Now the writer can update the head page. This is also why the head page must
+remain in UPDATE and only reset by the outermost writer. This prevents
+the reader from seeing the incorrect head page::
+
+
+  (first writer)
+
+                 A        B    tail page
+                 |        |        |
+                 v        v        v
+      +---+    +---+    +---+    +---+
+  <---|   |--->|   |--->|   |--->|   |-H->
+  --->|   |<---|   |<---|   |<---|   |<---
+      +---+    +---+    +---+    +---+