Merge branches 'doc.2021.01.06a', 'fixes.2021.01.04b',...

init_srcu_struct() APIs for defining and initializing

``srcu_struct`` structures.

More recently, the SRCU API has added polling interfaces:

#. start_poll_synchronize_srcu() returns a cookie identifying

   the completion of a future SRCU grace period and ensures

   that this grace period will be started.

#. poll_state_synchronize_srcu() returns ``true`` iff the

   specified cookie corresponds to an already-completed

   SRCU grace period.

#. get_state_synchronize_srcu() returns a cookie just like

   start_poll_synchronize_srcu() does, but differs in that

   it does nothing to ensure that any future SRCU grace period

   will be started.

These functions are used to avoid unnecessary SRCU grace periods in

certain types of buffer-cache algorithms having multi-stage age-out

mechanisms.  The idea is that by the time the block has aged completely

from the cache, an SRCU grace period will be very likely to have elapsed.

Tasks RCU

~~~~~~~~~

	buggy timer hardware through bugs in the interrupt or exception

	path (whether hardware, firmware, or software) through bugs

	in Linux's timer subsystem through bugs in the scheduler, and,

	yes, even including bugs in RCU itself.

	yes, even including bugs in RCU itself.  It can also result in

	the ``rcu_.*timer wakeup didn't happen for`` console-log message,

	which will include additional debugging information.

-	A bug in the RCU implementation.

task_struct ->state field, and the "cpu" indicates that the grace-period

kthread last ran on CPU 5.

If the relevant grace-period kthread does not wake from FQS wait in a

reasonable time, then the following additional line is printed::

	kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402

The "23804" indicates that kthread's timer expired more than 23 thousand

jiffies ago.  The rest of the line has meaning similar to the kthread

starvation case.

Additionally, the following line is printed::

	Possible timer handling issue on cpu=4 timer-softirq=11142

Here "cpu" indicates that the grace-period kthread last ran on CPU 4,

where it queued the fqs timer.  The number following the "timer-softirq"

is the current ``TIMER_SOFTIRQ`` count on cpu 4.  If this value does not

change on successive RCU CPU stall warnings, there is further reason to

suspect a timer problem.

Multiple Warnings From One Stall

================================

			value, meaning that RCU_SOFTIRQ is used by default.

			Specify rcutree.use_softirq=0 to use rcuc kthreads.

			But note that CONFIG_PREEMPT_RT=y kernels disable

			this kernel boot parameter, forcibly setting it

			to zero.

	rcutree.rcu_fanout_exact= [KNL]

			Disable autobalancing of the rcu_node combining

			tree.  This is used by rcutorture, and might

			stress RCU, they don't participate in the actual

			test, hence the "fake".

	rcutorture.nocbs_nthreads= [KNL]

			Set number of RCU callback-offload togglers.

			Zero (the default) disables toggling.

	rcutorture.nocbs_toggle= [KNL]

			Set the delay in milliseconds between successive

			callback-offload toggling attempts.

	rcutorture.nreaders= [KNL]

			Set number of RCU readers.  The value -1 selects

			N-1, where N is the number of CPUs.  A value

			only normal grace-period primitives.  No effect

			on CONFIG_TINY_RCU kernels.

			But note that CONFIG_PREEMPT_RT=y kernels enables

			this kernel boot parameter, forcibly setting

			it to the value one, that is, converting any

			post-boot attempt at an expedited RCU grace

			period to instead use normal non-expedited

			grace-period processing.

	rcupdate.rcu_task_ipi_delay= [KNL]

			Set time in jiffies during which RCU tasks will

			avoid sending IPIs, starting with the beginning

	refscale.verbose= [KNL]

			Enable additional printk() statements.

	refscale.verbose_batched= [KNL]

			Batch the additional printk() statements.  If zero

			(the default) or negative, print everything.  Otherwise,

			print every Nth verbose statement, where N is the value

			specified.

	relax_domain_level=

			[KNL, SMP] Set scheduler's default relax_domain_level.

			See Documentation/admin-guide/cgroup-v1/cpusets.rst.

			are running concurrently, especially on systems

			with rotating-rust storage.

	torture.verbose_sleep_frequency= [KNL]

			Specifies how many verbose printk()s should be

			emitted between each sleep.  The default of zero

			disables verbose-printk() sleeping.

	torture.verbose_sleep_duration= [KNL]

			Duration of each verbose-printk() sleep in jiffies.

	tp720=		[HW,PS2]

	tpm_suspend_pcr=[HW,TPM]

#endif /* CONFIG_SMP */

extern struct bus_type cpu_subsys;

extern int lockdep_is_cpus_held(void);

#ifdef CONFIG_HOTPLUG_CPU

extern void cpus_write_lock(void);

extern void cpus_write_unlock(void);

extern int sysctl_nr_trim_pages;

void mem_dump_obj(void *object);

#endif /* __KERNEL__ */

#endif /* _LINUX_MM_H */