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 <paulmck@linux.ibm.com>

1 files changed, 34 insertions, 42 deletions

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 6f469864d9f5..fcc59fea5cd4 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -182,16 +182,13 @@ over a rather long period of time, but improvements are always welcome!
 		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
@@ -212,20 +209,20 @@ over a rather long period of time, but improvements are always welcome!
 	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()
@@ -288,8 +285,7 @@ over a rather long period of time, but improvements are always welcome!
 	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
@@ -336,12 +332,12 @@ over a rather long period of time, but improvements are always welcome!
 	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()
@@ -381,8 +377,7 @@ over a rather long period of time, but improvements are always welcome!
 
 	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.
@@ -428,22 +423,19 @@ over a rather long period of time, but improvements are always welcome!
 	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