Pull RCU updates from Joel Fernandes:
 "NOCB CPU management:

   - Consolidate rcu_nocb_cpu_offload() and rcu_nocb_cpu_deoffload() to
     reduce code duplication

   - Extract nocb_bypass_needs_flush() helper to reduce duplication in
     NOCB bypass path

  rcutorture/torture infrastructure:

   - Add NOCB01 config for RCU_LAZY torture testing

   - Add NOCB02 config for NOCB poll mode testing

   - Add TRIVIAL-PREEMPT config for textbook-style preemptible RCU
     torture

   - Test call_srcu() with preemption both disabled and enabled

   - Remove kvm-check-branches.sh in favor of kvm-series.sh

   - Make hangs more visible in torture.sh output

   - Add informative message for tests without a recheck file

   - Fix numeric test comparison in srcu_lockdep.sh

   - Use torture_shutdown_init() in refscale and rcuscale instead of
     open-coded shutdown functions

   - Fix modulo-zero error in torture_hrtimeout_ns().

  SRCU:

   - Fix SRCU read flavor macro comments

   - Fix s/they disables/they disable/ typo in srcu_read_unlock_fast()

  RCU Tasks:

   - Document that RCU Tasks Trace grace periods now imply RCU grace
     periods

   - Remove unnecessary smp_store_release() in cblist_init_generic()"

* tag 'rcu.2026.03.31a' of git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux:
  rcutorture: Test call_srcu() with preemption disabled and not
  rcu: Add BOOTPARAM_RCU_STALL_PANIC Kconfig option
  torture: Avoid modulo-zero error in torture_hrtimeout_ns()
  rcu/nocb: Extract nocb_bypass_needs_flush() to reduce duplication
  rcu/nocb: Consolidate rcu_nocb_cpu_offload/deoffload functions
  rcu-tasks: Remove unnecessary smp_store_release() in cblist_init_generic()
  rcutorture: Add NOCB02 config for nocb poll mode testing
  rcutorture: Add NOCB01 config for RCU_LAZY torture testing
  rcu-tasks: Document that RCU Tasks Trace grace periods now imply RCU grace periods
  srcu: Fix s/they disables/they disable/ typo in srcu_read_unlock_fast()
  srcu: Fix SRCU read flavor macro comments
  rcuscale: Ditch rcu_scale_shutdown in favor of torture_shutdown_init()
  refscale: Ditch ref_scale_shutdown in favor of torture_shutdown_init()
  rcutorture: Fix numeric "test" comparison in srcu_lockdep.sh
  torture: Print informative message for test without recheck file
  torture: Make hangs more visible in torture.sh output
  kvm-check-branches.sh: Remove in favor of kvm-series.sh
  rcutorture: Add a textbook-style trivial preemptible RCU

Add a Kconfig option to set the default value of the
kernel.panic_on_rcu_stall sysctl, allowing the kernel to be built
with panic-on-RCU-stall enabled by default.

This is useful for high-availability systems that require automatic
recovery (via panic_timeout) when a CPU stall is detected, without
needing userspace to configure the sysctl at boot.

This follows the pattern established by BOOTPARAM_SOFTLOCKUP_PANIC
and BOOTPARAM_HUNG_TASK_PANIC.  The runtime sysctl can still override
the Kconfig default.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

This commit adds a trivial textbook implementation of preemptible RCU
to rcutorture ("torture_type=trivial-preempt"), similar in spirit to the
existing "torture_type=trivial" textbook implementation of non-preemptible
RCU.  Neither trivial RCU implementation has any value for production use,
and are intended only to keep Paul honest in his introductory writings
and presentations.

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

A later commit will reduce the size of the RCU watching counter to free up
some bits for another purpose. Paul suggested adding a config option to
test the extreme case where the counter is reduced to its minimum usable
width for rcutorture to poke at, so do that.

Make it only configurable under RCU_EXPERT. While at it, add a comment to
explain the layout of context_tracking->state.

Link: http://lore.kernel.org/r/4c2cb573-168f-4806-b1d9-164e8276e66a@paulmck-laptop
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

The RCU_TORTURE_TEST_CHK_RDR_STATE and RCU_TORTURE_TEST_LOG_CPU Kconfig
options are pointlessly defined as tristate.  This commit therefore
converts them to bool.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202412241458.150d082b-lkp@intel.com
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

This commit includes the grace-period sequence numbers at the beginning
and end of each segment in the "Failure/close-call rcutorture reader
segments" list.  These are in hexadecimal, and only the bottom byte.
Currently, only RCU is supported, with its three sequence numbers (normal,
expedited, and polled).

Note that if all the grace-period sequence numbers remain the same across
a given reader segment, only one copy of the number will be printed.
Of course, if there is a change, both sets of values will be printed.

Because the overhead of collecting this information can suppress
heisenbugs, this information is collected and printed only in kernels
built with CONFIG_RCU_TORTURE_TEST_LOG_GP=y.

[ paulmck: Apply Nathan Chancellor feedback for IS_ENABLED(). ]
[ paulmck: Apply feedback from kernel test robot. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

This commit adds reader-state debugging checks to a new function named
rcutorture_one_extend_check(), which is invoked before and after setting
new reader states by the existing rcutorture_one_extend() function.
These checks have proven to be rather heavyweight, reducing reproduction
rate of some failures by a factor of two.  They are therefore hidden
behind a new RCU_TORTURE_TEST_CHK_RDR_STATE Kconfig option.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

This commit adds CPU number to the "Failure/close-call rcutorture reader
segments" list printed at the end of an rcutorture run that had too-short
grace periods.  This information can help debugging interactions with
migration and CPU hotplug.

However, experience indicates that sampling the CPU number in rcutorture's
read-side code can reduce the probability of too-short bugs by a small
integer factor.  And small integer factors are crucial to RCU bug hunting,
so this commit also introduces a default-off RCU_TORTURE_TEST_LOG_CPU
Kconfig option to enable this CPU-number-logging functionality at
build time.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Although the RCU CPU stall notifiers can be useful for dumping state when
tracking down delicate forward-progress bugs where NUMA effects cause
cache lines to be delivered to a given CPU regularly, but always in a
state that prevents that CPU from making forward progress.  These bugs can
be detected by the RCU CPU stall-warning mechanism, but in some cases,
the stall-warnings printk()s disrupt the forward-progress bug before
any useful state can be obtained.

Unfortunately, the notifier mechanism added by commit 5b404fdabacf ("rcu:
Add RCU CPU stall notifier") can make matters worse if used at all
carelessly. For example, if the stall warning was caused by a lock not
being released, then any attempt to acquire that lock in the notifier
will hang. This will prevent not only the notifier from producing any
useful output, but it will also prevent the stall-warning message from
ever appearing.

This commit therefore hides this new RCU CPU stall notifier
mechanism under a new RCU_CPU_STALL_NOTIFIER Kconfig option that
depends on both DEBUG_KERNEL and RCU_EXPERT.  In addition, the
rcupdate.rcu_cpu_stall_notifiers=1 kernel boot parameter must also
be specified.  The RCU_CPU_STALL_NOTIFIER Kconfig option's help text
contains a warning and explains the dangers of careless use, recommending
lockless notifier code.  In addition, a WARN() is triggered each time
that an attempt is made to register a stall-warning notifier in kernels
built with CONFIG_RCU_CPU_STALL_NOTIFIER=y.

This combination of measures will keep use of this mechanism confined to
debug kernels and away from routine deployments.

[ paulmck: Apply Dan Carpenter feedback. ]

Fixes: 5b404fdabacf ("rcu: Add RCU CPU stall notifier")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>

The maximum value of RCU CPU stall-warning timeouts has historically been
five minutes (300 seconds).  However, the recently introduced expedited
RCU CPU stall-warning timeout is instead limited to 21 seconds.  This
causes problems for CI/fuzzing services such as syzkaller by obscuring
the issue in question with expedited RCU CPU stall-warning timeout splats.

This commit therefore sets the RCU_EXP_CPU_STALL_TIMEOUT Kconfig options
upper bound to 300000 milliseconds, which is 300 seconds (AKA 5 minutes).

[ paulmck: Apply feedback from Hillf Danton. ]
[ paulmck: Apply feedback from Geert Uytterhoeven. ]

Reported-by: Dave Chinner <david@fromorbit.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>