doc: Remove obsolete RCU update functions from RCU documentation

Now that synchronize_rcu_bh, synchronize_rcu_bh_expedited, call_rcu_bh,
rcu_barrier_bh, synchronize_sched, synchronize_sched_expedited,
call_rcu_sched, rcu_barrier_sched, get_state_synchronize_sched,
and cond_synchronize_sched are obsolete, let's remove them from the
documentation aside from a small historical section.

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

Documentation/RCU/Design/Data-Structures/Data-Structures.html

···
 of the level of loading on the system.
 
 </p><p>RCU updaters wait for normal grace periods by registering
-RCU callbacks, either directly via <tt>call_rcu()</tt> and
-friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
+RCU callbacks, either directly via <tt>call_rcu()</tt>
 or indirectly via <tt>synchronize_rcu()</tt> and friends.
 RCU callbacks are represented by <tt>rcu_head</tt> structures,
 which are queued on <tt>rcu_data</tt> structures while they are

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

···
 RCU-preempt Expedited Grace Periods</a></h2>
 
 <p>
+<tt>CONFIG_PREEMPT=y</tt> 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</a></h2>
 
 <p>
+<tt>CONFIG_PREEMPT=n</tt> 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:
 
···
 
 <p>
 As with RCU-preempt, RCU-sched's
-<tt>synchronize_sched_expedited()</tt> ignores offline and
+<tt>synchronize_rcu_expedited()</tt> ignores offline and
 idle CPUs, again because they are in remotely detectable
 quiescent states.
 However, because the

Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html

···
 period is guaranteed to see the effects of all accesses following the end
 of that grace period that are within RCU read-side critical sections.
 
-<p>This guarantee is particularly pervasive for <tt>synchronize_sched()</tt>,
-for which RCU-sched read-side critical sections include any region
+<p>Note well that RCU-sched read-side critical sections include any region
 of code for which preemption is disabled.
 Given that each individual machine instruction can be thought of as
 an extremely small region of preemption-disabled code, one can think of
-<tt>synchronize_sched()</tt> as <tt>smp_mb()</tt> on steroids.
+<tt>synchronize_rcu()</tt> as <tt>smp_mb()</tt> on steroids.
 
 <p>RCU updaters use this guarantee by splitting their updates into
 two phases, one of which is executed before the grace period and

Documentation/RCU/NMI-RCU.txt

···
 up any data structures used by the old NMI handler until execution
 of it completes on all other CPUs.
 
-One way to accomplish this is via synchronize_sched(), perhaps as
+One way to accomplish this is via synchronize_rcu(), perhaps as
 follows:
 
 	unset_nmi_callback();
-	synchronize_sched();
+	synchronize_rcu();
 	kfree(my_nmi_data);
 
-This works because synchronize_sched() blocks until all CPUs complete
-any preemption-disabled segments of code that they were executing.
-Since NMI handlers disable preemption, synchronize_sched() is guaranteed
+This works because (as of v4.20) synchronize_rcu() blocks until all
+CPUs complete any preemption-disabled segments of code that they were
+executing.
+Since NMI handlers disable preemption, synchronize_rcu() is guaranteed
 not to return until all ongoing NMI handlers exit.  It is therefore safe
-to free up the handler's data as soon as synchronize_sched() returns.
+to free up the handler's data as soon as synchronize_rcu() returns.
 
 Important note: for this to work, the architecture in question must
 invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.

Documentation/RCU/UP.txt

···
 infrastructure -must- respect grace periods, and -must- invoke callbacks
 from a known environment in which no locks are held.
 
-It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return
-immediately on an UP system.  It is also safe for synchronize_rcu()
-to return immediately on UP systems, except when running preemptable
-RCU.
+Note that it -is- safe for synchronize_rcu() to return immediately on
+UP systems, including !PREEMPT SMP builds running on UP systems.
 
 Quick Quiz #3: Why can't synchronize_rcu() return immediately on
 	UP systems running preemptable RCU?

Documentation/RCU/checklist.txt

···
 		when publicizing a pointer to a structure that can
 		be traversed by an RCU read-side critical section.
 
-5.	If call_rcu(), or a related primitive such as call_rcu_bh(),
-	call_rcu_sched(), or call_srcu() is used, the callback function
-	will be called from softirq context.  In particular, it cannot
-	block.
+5.	If call_rcu() or call_srcu() is used, the callback function will
+	be called from softirq context.  In particular, it cannot block.
 
-6.	Since synchronize_rcu() can block, it cannot be called from
-	any sort of irq context.  The same rule applies for
-	synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
-	synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
-	synchronize_sched_expedite(), and synchronize_srcu_expedited().
+6.	Since synchronize_rcu() can block, it cannot be called
+	from any sort of irq context.  The same rule applies
+	for synchronize_srcu(), synchronize_rcu_expedited(), and
+	synchronize_srcu_expedited().
 
 	The expedited forms of these primitives have the same semantics
 	as the non-expedited forms, but expediting is both expensive and
···
 	of the system, especially to real-time workloads running on
 	the rest of the system.
 
-7.	If the updater uses call_rcu() or synchronize_rcu(), then the
-	corresponding readers must use rcu_read_lock() and
-	rcu_read_unlock().  If the updater uses call_rcu_bh() or
-	synchronize_rcu_bh(), then the corresponding readers must
-	use rcu_read_lock_bh() and rcu_read_unlock_bh().  If the
-	updater uses call_rcu_sched() or synchronize_sched(), then
-	the corresponding readers must disable preemption, possibly
-	by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
-	If the updater uses synchronize_srcu() or call_srcu(), then
-	the corresponding readers must use srcu_read_lock() and
+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.
+	If the updater uses call_rcu() or synchronize_rcu(),
+	then the corresponding readers my use rcu_read_lock() and
+	rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(),
+	or any pair of primitives that disables and re-enables preemption,
+	for example, rcu_read_lock_sched() and rcu_read_unlock_sched().
+	If the updater uses synchronize_srcu() or call_srcu(),
+	then the corresponding readers must use srcu_read_lock() and
 	srcu_read_unlock(), and with the same srcu_struct.  The rules for
 	the expedited primitives are the same as for their non-expedited
 	counterparts.  Mixing things up will result in confusion and
-	broken kernels.
+	broken kernels, and has even resulted in an exploitable security
+	issue.
 
 	One exception to this rule: rcu_read_lock() and rcu_read_unlock()
 	may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
···
 	d.	Periodically invoke synchronize_rcu(), permitting a limited
 		number of updates per grace period.
 
-	The same cautions apply to call_rcu_bh(), call_rcu_sched(),
-	call_srcu(), and kfree_rcu().
+	The same cautions apply to call_srcu() and kfree_rcu().
 
 	Note that although these primitives do take action to avoid memory
 	exhaustion when any given CPU has too many callbacks, a determined
···
 	to safely access and/or modify that data structure.
 
 	RCU callbacks are -usually- executed on the same CPU that executed
-	the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
-	but are by -no- means guaranteed to be.  For example, if a given
-	CPU goes offline while having an RCU callback pending, then that
-	RCU callback will execute on some surviving CPU.  (If this was
-	not the case, a self-spawning RCU callback would prevent the
-	victim CPU from ever going offline.)
+	the corresponding call_rcu() or call_srcu().  but are by -no-
+	means guaranteed to be.  For example, if a given CPU goes offline
+	while having an RCU callback pending, then that RCU callback
+	will execute on some surviving CPU.  (If this was not the case,
+	a self-spawning RCU callback would prevent the victim CPU from
+	ever going offline.)
 
 13.	Unlike other forms of RCU, it -is- permissible to block in an
 	SRCU read-side critical section (demarked by srcu_read_lock()
···
 
 	SRCU's expedited primitive (synchronize_srcu_expedited())
 	never sends IPIs to other CPUs, so it is easier on
-	real-time workloads than is synchronize_rcu_expedited(),
-	synchronize_rcu_bh_expedited() or synchronize_sched_expedited().
+	real-time workloads than is synchronize_rcu_expedited().
 
 	Note that rcu_dereference() and rcu_assign_pointer() relate to
 	SRCU just as they do to other forms of RCU.
···
 	These debugging aids can help you find problems that are
 	otherwise extremely difficult to spot.
 
-17.	If you register a callback using call_rcu(), call_rcu_bh(),
-	call_rcu_sched(), or call_srcu(), and pass in a function defined
-	within a loadable module, then it in necessary to wait for
-	all pending callbacks to be invoked after the last invocation
-	and before unloading that module.  Note that it is absolutely
-	-not- sufficient to wait for a grace period!  The current (say)
-	synchronize_rcu() implementation waits only for all previous
-	callbacks registered on the CPU that synchronize_rcu() is running
-	on, but it is -not- guaranteed to wait for callbacks registered
-	on other CPUs.
+17.	If you register a callback using call_rcu() or call_srcu(),
+	and pass in a function defined within a loadable module,
+	then it in necessary to wait for all pending callbacks to
+	be invoked after the last invocation and before unloading
+	that module.  Note that it is absolutely -not- sufficient to
+	wait for a grace period!  The current (say) synchronize_rcu()
+	implementation waits only for all previous callbacks registered
+	on the CPU that synchronize_rcu() is running on, but it is -not-
+	guaranteed to wait for callbacks registered on other CPUs.
 
 	You instead need to use one of the barrier functions:
 
 	o	call_rcu() -> rcu_barrier()
-	o	call_rcu_bh() -> rcu_barrier()
-	o	call_rcu_sched() -> rcu_barrier()
 	o	call_srcu() -> srcu_barrier()
 
 	However, these barrier functions are absolutely -not- guaranteed

Documentation/RCU/rcu.txt

···
 o	How can I see where RCU is currently used in the Linux kernel?
 
 	Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
-	"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
-	"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
-	"synchronize_net", "synchronize_srcu", and the other RCU
-	primitives.  Or grab one of the cscope databases from:
+	"rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",
+	"srcu_read_unlock", "synchronize_rcu", "synchronize_net",
+	"synchronize_srcu", and the other RCU primitives.  Or grab one
+	of the cscope databases from:
 
 	http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
 

Documentation/RCU/rcubarrier.txt

···
    2. Execute rcu_barrier().
    3. Allow the module to be unloaded.
 
-There are also rcu_barrier_bh(), rcu_barrier_sched(), and srcu_barrier()
-functions for the other flavors of RCU, and you of course must match
-the flavor of rcu_barrier() with that of call_rcu().  If your module
-uses multiple flavors of call_rcu(), then it must also use multiple
+There is also an srcu_barrier() function for SRCU, and you of course
+must match the flavor of rcu_barrier() with that of call_rcu().  If your
+module uses multiple flavors of call_rcu(), then it must also use multiple
 flavors of rcu_barrier() when unloading that module.  For example, if
-it uses call_rcu_bh(), call_srcu() on srcu_struct_1, and call_srcu() on
+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_bh();
+ 1 rcu_barrier();
  2 srcu_barrier(&srcu_struct_1);
  3 srcu_barrier(&srcu_struct_2);
 
···
 the timers, and only then invoke rcu_barrier() to wait for any remaining
 RCU callbacks to complete.
 
-Of course, if you module uses call_rcu_bh(), you will need to invoke
-rcu_barrier_bh() before unloading.  Similarly, if your module uses
-call_rcu_sched(), you will need to invoke rcu_barrier_sched() before
-unloading.  If your module uses call_rcu(), call_rcu_bh(), -and-
-call_rcu_sched(), then you will need to invoke each of rcu_barrier(),
-rcu_barrier_bh(), and rcu_barrier_sched().
+Of course, if you 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 you will need to invoke rcu_barrier() -and-
+srcu_barrier().
 
 
 Implementing rcu_barrier()
···
 ensures that all the calls to rcu_barrier_func() will have completed
 before on_each_cpu() returns. Line 9 then waits for the completion.
 
-This code was rewritten in 2008 to support rcu_barrier_bh() and
-rcu_barrier_sched() in addition to the original rcu_barrier().
+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: