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, 380 insertions, 0 deletions

diff --git a/Documentation/RCU/rcubarrier.rst b/Documentation/RCU/rcubarrier.rst
new file mode 100644
index 000000000..6da7f66da
--- /dev/null
+++ b/Documentation/RCU/rcubarrier.rst
@@ -0,0 +1,380 @@
+.. _rcu_barrier:
+
+RCU and Unloadable Modules
+==========================
+
+[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
+
+RCU updaters sometimes use call_rcu() to initiate an asynchronous wait for
+a grace period to elapse.  This primitive takes a pointer to an rcu_head
+struct placed within the RCU-protected data structure and another pointer
+to a function that may be invoked later to free that structure. Code to
+delete an element p from the linked list from IRQ context might then be
+as follows::
+
+	list_del_rcu(p);
+	call_rcu(&p->rcu, p_callback);
+
+Since call_rcu() never blocks, this code can safely be used from within
+IRQ context. The function p_callback() might be defined as follows::
+
+	static void p_callback(struct rcu_head *rp)
+	{
+		struct pstruct *p = container_of(rp, struct pstruct, rcu);
+
+		kfree(p);
+	}
+
+
+Unloading Modules That Use call_rcu()
+-------------------------------------
+
+But what if the p_callback() function is defined in an unloadable module?
+
+If we unload the module while some RCU callbacks are pending,
+the CPUs executing these callbacks are going to be severely
+disappointed when they are later invoked, as fancifully depicted at
+http://lwn.net/images/ns/kernel/rcu-drop.jpg.
+
+We could try placing a synchronize_rcu() in the module-exit code path,
+but this is not sufficient. Although synchronize_rcu() does wait for a
+grace period to elapse, it does not wait for the callbacks to complete.
+
+One might be tempted to try several back-to-back synchronize_rcu()
+calls, but this is still not guaranteed to work. If there is a very
+heavy RCU-callback load, then some of the callbacks might be deferred in
+order to allow other processing to proceed. For but one example, such
+deferral is required in realtime kernels in order to avoid excessive
+scheduling latencies.
+
+
+rcu_barrier()
+-------------
+
+This situation can be handled by the rcu_barrier() primitive.  Rather
+than waiting for a grace period to elapse, rcu_barrier() waits for all
+outstanding RCU callbacks to complete.  Please note that rcu_barrier()
+does **not** imply synchronize_rcu(), in particular, if there are no RCU
+callbacks queued anywhere, rcu_barrier() is within its rights to return
+immediately, without waiting for anything, let alone a grace period.
+
+Pseudo-code using rcu_barrier() is as follows:
+
+   1. Prevent any new RCU callbacks from being posted.
+   2. Execute rcu_barrier().
+   3. Allow the module to be unloaded.
+
+There is also an srcu_barrier() function for SRCU, and you of course
+must match the flavor of srcu_barrier() with that of call_srcu().
+If your module uses multiple srcu_struct structures, then it must also
+use multiple invocations of srcu_barrier() when unloading that module.
+For example, if it uses call_rcu(), call_srcu() on srcu_struct_1, and
+call_srcu() on srcu_struct_2, then the following three lines of code
+will be required when unloading::
+
+  1  rcu_barrier();
+  2  srcu_barrier(&srcu_struct_1);
+  3  srcu_barrier(&srcu_struct_2);
+
+If latency is of the essence, workqueues could be used to run these
+three functions concurrently.
+
+An ancient version of the rcutorture module makes use of rcu_barrier()
+in its exit function as follows::
+
+  1  static void
+  2  rcu_torture_cleanup(void)
+  3  {
+  4    int i;
+  5
+  6    fullstop = 1;
+  7    if (shuffler_task != NULL) {
+  8      VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
+  9      kthread_stop(shuffler_task);
+ 10    }
+ 11    shuffler_task = NULL;
+ 12
+ 13    if (writer_task != NULL) {
+ 14      VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
+ 15      kthread_stop(writer_task);
+ 16    }
+ 17    writer_task = NULL;
+ 18
+ 19    if (reader_tasks != NULL) {
+ 20      for (i = 0; i < nrealreaders; i++) {
+ 21        if (reader_tasks[i] != NULL) {
+ 22          VERBOSE_PRINTK_STRING(
+ 23            "Stopping rcu_torture_reader task");
+ 24          kthread_stop(reader_tasks[i]);
+ 25        }
+ 26        reader_tasks[i] = NULL;
+ 27      }
+ 28      kfree(reader_tasks);
+ 29      reader_tasks = NULL;
+ 30    }
+ 31    rcu_torture_current = NULL;
+ 32
+ 33    if (fakewriter_tasks != NULL) {
+ 34      for (i = 0; i < nfakewriters; i++) {
+ 35        if (fakewriter_tasks[i] != NULL) {
+ 36          VERBOSE_PRINTK_STRING(
+ 37            "Stopping rcu_torture_fakewriter task");
+ 38          kthread_stop(fakewriter_tasks[i]);
+ 39        }
+ 40        fakewriter_tasks[i] = NULL;
+ 41      }
+ 42      kfree(fakewriter_tasks);
+ 43      fakewriter_tasks = NULL;
+ 44    }
+ 45
+ 46    if (stats_task != NULL) {
+ 47      VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
+ 48      kthread_stop(stats_task);
+ 49    }
+ 50    stats_task = NULL;
+ 51
+ 52    /* Wait for all RCU callbacks to fire. */
+ 53    rcu_barrier();
+ 54
+ 55    rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
+ 56
+ 57    if (cur_ops->cleanup != NULL)
+ 58      cur_ops->cleanup();
+ 59    if (atomic_read(&n_rcu_torture_error))
+ 60      rcu_torture_print_module_parms("End of test: FAILURE");
+ 61    else
+ 62      rcu_torture_print_module_parms("End of test: SUCCESS");
+ 63  }
+
+Line 6 sets a global variable that prevents any RCU callbacks from
+re-posting themselves. This will not be necessary in most cases, since
+RCU callbacks rarely include calls to call_rcu(). However, the rcutorture
+module is an exception to this rule, and therefore needs to set this
+global variable.
+
+Lines 7-50 stop all the kernel tasks associated with the rcutorture
+module. Therefore, once execution reaches line 53, no more rcutorture
+RCU callbacks will be posted. The rcu_barrier() call on line 53 waits
+for any pre-existing callbacks to complete.
+
+Then lines 55-62 print status and do operation-specific cleanup, and
+then return, permitting the module-unload operation to be completed.
+
+.. _rcubarrier_quiz_1:
+
+Quick Quiz #1:
+	Is there any other situation where rcu_barrier() might
+	be required?
+
+:ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>`
+
+Your module might have additional complications. For example, if your
+module invokes call_rcu() from timers, you will need to first refrain
+from posting new timers, cancel (or wait for) all the already-posted
+timers, and only then invoke rcu_barrier() to wait for any remaining
+RCU callbacks to complete.
+
+Of course, if your module uses call_rcu(), you will need to invoke
+rcu_barrier() before unloading.  Similarly, if your module uses
+call_srcu(), you will need to invoke srcu_barrier() before unloading,
+and on the same srcu_struct structure.  If your module uses call_rcu()
+**and** call_srcu(), then (as noted above) you will need to invoke
+rcu_barrier() **and** srcu_barrier().
+
+
+Implementing rcu_barrier()
+--------------------------
+
+Dipankar Sarma's implementation of rcu_barrier() makes use of the fact
+that RCU callbacks are never reordered once queued on one of the per-CPU
+queues. His implementation queues an RCU callback on each of the per-CPU
+callback queues, and then waits until they have all started executing, at
+which point, all earlier RCU callbacks are guaranteed to have completed.
+
+The original code for rcu_barrier() was roughly as follows::
+
+  1  void rcu_barrier(void)
+  2  {
+  3    BUG_ON(in_interrupt());
+  4    /* Take cpucontrol mutex to protect against CPU hotplug */
+  5    mutex_lock(&rcu_barrier_mutex);
+  6    init_completion(&rcu_barrier_completion);
+  7    atomic_set(&rcu_barrier_cpu_count, 1);
+  8    on_each_cpu(rcu_barrier_func, NULL, 0, 1);
+  9    if (atomic_dec_and_test(&rcu_barrier_cpu_count))
+ 10      complete(&rcu_barrier_completion);
+ 11    wait_for_completion(&rcu_barrier_completion);
+ 12    mutex_unlock(&rcu_barrier_mutex);
+ 13  }
+
+Line 3 verifies that the caller is in process context, and lines 5 and 12
+use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
+global completion and counters at a time, which are initialized on lines
+6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is
+shown below. Note that the final "1" in on_each_cpu()'s argument list
+ensures that all the calls to rcu_barrier_func() will have completed
+before on_each_cpu() returns. Line 9 removes the initial count from
+rcu_barrier_cpu_count, and if this count is now zero, line 10 finalizes
+the completion, which prevents line 11 from blocking.  Either way,
+line 11 then waits (if needed) for the completion.
+
+.. _rcubarrier_quiz_2:
+
+Quick Quiz #2:
+	Why doesn't line 8 initialize rcu_barrier_cpu_count to zero,
+	thereby avoiding the need for lines 9 and 10?
+
+:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
+
+This code was rewritten in 2008 and several times thereafter, but this
+still gives the general idea.
+
+The rcu_barrier_func() runs on each CPU, where it invokes call_rcu()
+to post an RCU callback, as follows::
+
+  1  static void rcu_barrier_func(void *notused)
+  2  {
+  3    int cpu = smp_processor_id();
+  4    struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
+  5    struct rcu_head *head;
+  6
+  7    head = &rdp->barrier;
+  8    atomic_inc(&rcu_barrier_cpu_count);
+  9    call_rcu(head, rcu_barrier_callback);
+ 10  }
+
+Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
+which contains the struct rcu_head that needed for the later call to
+call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line
+8 increments the global counter. This counter will later be decremented
+by the callback. Line 9 then registers the rcu_barrier_callback() on
+the current CPU's queue.
+
+The rcu_barrier_callback() function simply atomically decrements the
+rcu_barrier_cpu_count variable and finalizes the completion when it
+reaches zero, as follows::
+
+  1  static void rcu_barrier_callback(struct rcu_head *notused)
+  2  {
+  3    if (atomic_dec_and_test(&rcu_barrier_cpu_count))
+  4      complete(&rcu_barrier_completion);
+  5  }
+
+.. _rcubarrier_quiz_3:
+
+Quick Quiz #3:
+	What happens if CPU 0's rcu_barrier_func() executes
+	immediately (thus incrementing rcu_barrier_cpu_count to the
+	value one), but the other CPU's rcu_barrier_func() invocations
+	are delayed for a full grace period? Couldn't this result in
+	rcu_barrier() returning prematurely?
+
+:ref:`Answer to Quick Quiz #3 <answer_rcubarrier_quiz_3>`
+
+The current rcu_barrier() implementation is more complex, due to the need
+to avoid disturbing idle CPUs (especially on battery-powered systems)
+and the need to minimally disturb non-idle CPUs in real-time systems.
+In addition, a great many optimizations have been applied.  However,
+the code above illustrates the concepts.
+
+
+rcu_barrier() Summary
+---------------------
+
+The rcu_barrier() primitive is used relatively infrequently, since most
+code using RCU is in the core kernel rather than in modules. However, if
+you are using RCU from an unloadable module, you need to use rcu_barrier()
+so that your module may be safely unloaded.
+
+
+Answers to Quick Quizzes
+------------------------
+
+.. _answer_rcubarrier_quiz_1:
+
+Quick Quiz #1:
+	Is there any other situation where rcu_barrier() might
+	be required?
+
+Answer:
+	Interestingly enough, rcu_barrier() was not originally
+	implemented for module unloading. Nikita Danilov was using
+	RCU in a filesystem, which resulted in a similar situation at
+	filesystem-unmount time. Dipankar Sarma coded up rcu_barrier()
+	in response, so that Nikita could invoke it during the
+	filesystem-unmount process.
+
+	Much later, yours truly hit the RCU module-unload problem when
+	implementing rcutorture, and found that rcu_barrier() solves
+	this problem as well.
+
+:ref:`Back to Quick Quiz #1 <rcubarrier_quiz_1>`
+
+.. _answer_rcubarrier_quiz_2:
+
+Quick Quiz #2:
+	Why doesn't line 8 initialize rcu_barrier_cpu_count to zero,
+	thereby avoiding the need for lines 9 and 10?
+
+Answer:
+	Suppose that the on_each_cpu() function shown on line 8 was
+	delayed, so that CPU 0's rcu_barrier_func() executed and
+	the corresponding grace period elapsed, all before CPU 1's
+	rcu_barrier_func() started executing.  This would result in
+	rcu_barrier_cpu_count being decremented to zero, so that line
+	11's wait_for_completion() would return immediately, failing to
+	wait for CPU 1's callbacks to be invoked.
+
+	Note that this was not a problem when the rcu_barrier() code
+	was first added back in 2005.  This is because on_each_cpu()
+	disables preemption, which acted as an RCU read-side critical
+	section, thus preventing CPU 0's grace period from completing
+	until on_each_cpu() had dealt with all of the CPUs.  However,
+	with the advent of preemptible RCU, rcu_barrier() no longer
+	waited on nonpreemptible regions of code in preemptible kernels,
+	that being the job of the new rcu_barrier_sched() function.
+
+	However, with the RCU flavor consolidation around v4.20, this
+	possibility was once again ruled out, because the consolidated
+	RCU once again waits on nonpreemptible regions of code.
+
+	Nevertheless, that extra count might still be a good idea.
+	Relying on these sort of accidents of implementation can result
+	in later surprise bugs when the implementation changes.
+
+:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
+
+.. _answer_rcubarrier_quiz_3:
+
+Quick Quiz #3:
+	What happens if CPU 0's rcu_barrier_func() executes
+	immediately (thus incrementing rcu_barrier_cpu_count to the
+	value one), but the other CPU's rcu_barrier_func() invocations
+	are delayed for a full grace period? Couldn't this result in
+	rcu_barrier() returning prematurely?
+
+Answer:
+	This cannot happen. The reason is that on_each_cpu() has its last
+	argument, the wait flag, set to "1". This flag is passed through
+	to smp_call_function() and further to smp_call_function_on_cpu(),
+	causing this latter to spin until the cross-CPU invocation of
+	rcu_barrier_func() has completed. This by itself would prevent
+	a grace period from completing on non-CONFIG_PREEMPTION kernels,
+	since each CPU must undergo a context switch (or other quiescent
+	state) before the grace period can complete. However, this is
+	of no use in CONFIG_PREEMPTION kernels.
+
+	Therefore, on_each_cpu() disables preemption across its call
+	to smp_call_function() and also across the local call to
+	rcu_barrier_func(). Because recent RCU implementations treat
+	preemption-disabled regions of code as RCU read-side critical
+	sections, this prevents grace periods from completing. This
+	means that all CPUs have executed rcu_barrier_func() before
+	the first rcu_barrier_callback() can possibly execute, in turn
+	preventing rcu_barrier_cpu_count from prematurely reaching zero.
+
+	But if on_each_cpu() ever decides to forgo disabling preemption,
+	as might well happen due to real-time latency considerations,
+	initializing rcu_barrier_cpu_count to one will save the day.
+
+:ref:`Back to Quick Quiz #3 <rcubarrier_quiz_3>`