doc: Use CONFIG_PREEMPTION

CONFIG_PREEMPTION is selected by CONFIG_PREEMPT and by CONFIG_PREEMPT_RT.
Both PREEMPT and PREEMPT_RT require the same functionality which today
depends on CONFIG_PREEMPT.

Update the documents and mention CONFIG_PREEMPTION. Spell out
CONFIG_PREEMPT_RT (instead PREEMPT_RT) since it is an option now.

Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst

···
 RCU-preempt Expedited Grace Periods
 ===================================
 
-``CONFIG_PREEMPT=y`` kernels implement RCU-preempt.
+``CONFIG_PREEMPTION=y`` kernels implement RCU-preempt.
 The overall flow of the handling of a given CPU by an RCU-preempt
 expedited grace period is shown in the following diagram:
 
···
 RCU-sched Expedited Grace Periods
 ---------------------------------
 
-``CONFIG_PREEMPT=n`` kernels implement RCU-sched. The overall flow of
+``CONFIG_PREEMPTION=n`` kernels implement RCU-sched. The overall flow of
 the handling of a given CPU by an RCU-sched expedited grace period is
 shown in the following diagram:
 

Documentation/RCU/Design/Requirements/Requirements.rst

···
 Production-quality implementations of rcu_read_lock() and
 rcu_read_unlock() are extremely lightweight, and in fact have
 exactly zero overhead in Linux kernels built for production use with
-``CONFIG_PREEMPT=n``.
+``CONFIG_PREEMPTION=n``.
 
 This guarantee allows ordering to be enforced with extremely low
 overhead to readers, for example:
···
 costs have plummeted. However, as I learned from Matt Mackall's
 `bloatwatch <http://elinux.org/Linux_Tiny-FAQ>`__ efforts, memory
 footprint is critically important on single-CPU systems with
-non-preemptible (``CONFIG_PREEMPT=n``) kernels, and thus `tiny
+non-preemptible (``CONFIG_PREEMPTION=n``) kernels, and thus `tiny
 RCU <https://lore.kernel.org/r/[e-mail]>`__
 was born. Josh Triplett has since taken over the small-memory banner
 with his `Linux kernel tinification <https://tiny.wiki.kernel.org/>`__
···
 
 Implementations of RCU for which rcu_read_lock() and
 rcu_read_unlock() generate no code, such as Linux-kernel RCU when
-``CONFIG_PREEMPT=n``, can be nested arbitrarily deeply. After all, there
+``CONFIG_PREEMPTION=n``, can be nested arbitrarily deeply. After all, there
 is no overhead. Except that if all these instances of
 rcu_read_lock() and rcu_read_unlock() are visible to the
 compiler, compilation will eventually fail due to exhausting memory,
···
 
 However, once the scheduler has spawned its first kthread, this early
 boot trick fails for synchronize_rcu() (as well as for
-synchronize_rcu_expedited()) in ``CONFIG_PREEMPT=y`` kernels. The
+synchronize_rcu_expedited()) in ``CONFIG_PREEMPTION=y`` kernels. The
 reason is that an RCU read-side critical section might be preempted,
 which means that a subsequent synchronize_rcu() really does have to
 wait for something, as opposed to simply returning immediately.
···
        5 rcu_read_unlock();
        6 do_something_with(v, user_v);
 
-If the compiler did make this transformation in a ``CONFIG_PREEMPT=n`` kernel
+If the compiler did make this transformation in a ``CONFIG_PREEMPTION=n`` kernel
 build, and if get_user() did page fault, the result would be a quiescent
 state in the middle of an RCU read-side critical section.  This misplaced
 quiescent state could result in line 4 being a use-after-free access,
···
 patchset <https://wiki.linuxfoundation.org/realtime/>`__. The
 real-time-latency response requirements are such that the traditional
 approach of disabling preemption across RCU read-side critical sections
-is inappropriate. Kernels built with ``CONFIG_PREEMPT=y`` therefore use
+is inappropriate. Kernels built with ``CONFIG_PREEMPTION=y`` therefore use
 an RCU implementation that allows RCU read-side critical sections to be
 preempted. This requirement made its presence known after users made it
 clear that an earlier `real-time
···
 RCU read-side critical section can be a quiescent state. Therefore,
 *RCU-sched* was created, which follows “classic” RCU in that an
 RCU-sched grace period waits for pre-existing interrupt and NMI
-handlers. In kernels built with ``CONFIG_PREEMPT=n``, the RCU and
+handlers. In kernels built with ``CONFIG_PREEMPTION=n``, the RCU and
 RCU-sched APIs have identical implementations, while kernels built with
-``CONFIG_PREEMPT=y`` provide a separate implementation for each.
+``CONFIG_PREEMPTION=y`` provide a separate implementation for each.
 
-Note well that in ``CONFIG_PREEMPT=y`` kernels,
+Note well that in ``CONFIG_PREEMPTION=y`` kernels,
 rcu_read_lock_sched() and rcu_read_unlock_sched() disable and
 re-enable preemption, respectively. This means that if there was a
 preemption attempt during the RCU-sched read-side critical section,
···
 
 The tasks-RCU API is quite compact, consisting only of
 call_rcu_tasks(), synchronize_rcu_tasks(), and
-rcu_barrier_tasks(). In ``CONFIG_PREEMPT=n`` kernels, trampolines
+rcu_barrier_tasks(). In ``CONFIG_PREEMPTION=n`` kernels, trampolines
 cannot be preempted, so these APIs map to call_rcu(),
 synchronize_rcu(), and rcu_barrier(), respectively. In
-``CONFIG_PREEMPT=y`` kernels, trampolines can be preempted, and these
+``CONFIG_PREEMPTION=y`` kernels, trampolines can be preempted, and these
 three APIs are therefore implemented by separate functions that check
 for voluntary context switches.
 

Documentation/RCU/checklist.rst

···
 	the rest of the system.
 
 7.	As of v4.20, a given kernel implements only one RCU flavor,
-	which is RCU-sched for PREEMPT=n and RCU-preempt for PREEMPT=y.
+	which is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
 	If the updater uses call_rcu() or synchronize_rcu(),
 	then the corresponding readers may use rcu_read_lock() and
 	rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(),

Documentation/RCU/rcubarrier.rst

···
 of as a replacement for read-writer locking (among other things), but with
 very low-overhead readers that are immune to deadlock, priority inversion,
 and unbounded latency. RCU read-side critical sections are delimited
-by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT
+by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPTION
 kernels, generate no code whatsoever.
 
 This means that RCU writers are unaware of the presence of concurrent
···
 	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_PREEMPT kernels,
+	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_PREEMPT kernels.
+	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

Documentation/RCU/stallwarn.rst

···
 
 -	A CPU looping with bottom halves disabled.
 
--	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
+-	For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel
 	without invoking schedule().  If the looping in the kernel is
 	really expected and desirable behavior, you might need to add
 	some calls to cond_resched().
···
 	result in the ``rcu_.*kthread starved for`` console-log message,
 	which will include additional debugging information.
 
--	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
+-	A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might
 	happen to preempt a low-priority task in the middle of an RCU
 	read-side critical section.   This is especially damaging if
 	that low-priority task is not permitted to run on any other CPU,

Documentation/RCU/whatisRCU.rst

···
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 This section presents a "toy" RCU implementation that is based on
 "classic RCU".  It is also short on performance (but only for updates) and
-on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
+on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION
 kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
 are the same as those shown in the preceding section, so they are omitted.
 ::
···
 Quick Quiz #3:
 		If it is illegal to block in an RCU read-side
 		critical section, what the heck do you do in
-		PREEMPT_RT, where normal spinlocks can block???
+		CONFIG_PREEMPT_RT, where normal spinlocks can block???
 
 :ref:`Answers to Quick Quiz <8_whatisRCU>`
 
···
 		overhead is **negative**.
 
 Answer:
-		Imagine a single-CPU system with a non-CONFIG_PREEMPT
+		Imagine a single-CPU system with a non-CONFIG_PREEMPTION
 		kernel where a routing table is used by process-context
 		code, but can be updated by irq-context code (for example,
 		by an "ICMP REDIRECT" packet).	The usual way of handling
···
 Quick Quiz #3:
 		If it is illegal to block in an RCU read-side
 		critical section, what the heck do you do in
-		PREEMPT_RT, where normal spinlocks can block???
+		CONFIG_PREEMPT_RT, where normal spinlocks can block???
 
 Answer:
-		Just as PREEMPT_RT permits preemption of spinlock
+		Just as CONFIG_PREEMPT_RT permits preemption of spinlock
 		critical sections, it permits preemption of RCU
 		read-side critical sections.  It also permits
 		spinlocks blocking while in RCU read-side critical