rcu: Update comments and help text for no more RCU-bh updaters

This commit updates comments and help text to account for the fact that
RCU-bh update-side functions are now simple wrappers for their RCU or
RCU-sched counterparts.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcupdate.h

@@ -688,14 +688,10 @@ static inline void rcu_read_unlock(void)
 /**
  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
  *
- * This is equivalent of rcu_read_lock(), but to be used when updates
- * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
- * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
- * softirq handler to be a quiescent state, a process in RCU read-side
- * critical section must be protected by disabling softirqs. Read-side
- * critical sections in interrupt context can use just rcu_read_lock(),
- * though this should at least be commented to avoid confusing people
- * reading the code.
+ * This is equivalent of rcu_read_lock(), but also disables softirqs.
+ * Note that synchronize_rcu() and friends may be used for the update
+ * side, although synchronize_rcu_bh() is available as a wrapper in the
+ * short term.  Longer term, the _bh update-side API will be eliminated.
  *
  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
  * must occur in the same context, for example, it is illegal to invoke

include/linux/rcupdate_wait.h

@@ -36,13 +36,13 @@ do {									\
  * @...: List of call_rcu() functions for the flavors to wait on.
  *
  * This macro waits concurrently for multiple flavors of RCU grace periods.
- * For example, synchronize_rcu_mult(call_rcu, call_rcu_bh) would wait
- * on concurrent RCU and RCU-bh grace periods.  Waiting on a give SRCU
+ * For example, synchronize_rcu_mult(call_rcu, call_rcu_sched) would wait
+ * on concurrent RCU and RCU-sched grace periods.  Waiting on a give SRCU
  * domain requires you to write a wrapper function for that SRCU domain's
  * call_srcu() function, supplying the corresponding srcu_struct.
  *
  * If Tiny RCU, tell _wait_rcu_gp() not to bother waiting for RCU
- * or RCU-bh, given that anywhere synchronize_rcu_mult() can be called
+ * or RCU-sched, given that anywhere synchronize_rcu_mult() can be called
  * is automatically a grace period.
  */
 #define synchronize_rcu_mult(...) \

include/linux/rcutree.h

@@ -58,18 +58,8 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
 /**
  * synchronize_rcu_bh_expedited - Brute-force RCU-bh grace period
  *
- * Wait for an RCU-bh grace period to elapse, but use a "big hammer"
- * approach to force the grace period to end quickly.  This consumes
- * significant time on all CPUs and is unfriendly to real-time workloads,
- * so is thus not recommended for any sort of common-case code.  In fact,
- * if you are using synchronize_rcu_bh_expedited() in a loop, please
- * restructure your code to batch your updates, and then use a single
- * synchronize_rcu_bh() instead.
- *
- * Note that it is illegal to call this function while holding any lock
- * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
- * to call this function from a CPU-hotplug notifier.  Failing to observe
- * these restriction will result in deadlock.
+ * This is a transitional API and will soon be removed, with all
+ * callers converted to synchronize_rcu_expedited().
  */
 static inline void synchronize_rcu_bh_expedited(void)
 {

kernel/rcu/Kconfig

@@ -229,11 +229,11 @@ config RCU_NOCB_CPU
 	  CPUs specified at boot time by the rcu_nocbs parameter.
 	  For each such CPU, a kthread ("rcuox/N") will be created to
 	  invoke callbacks, where the "N" is the CPU being offloaded,
-	  and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and
-	  "s" for RCU-sched.  Nothing prevents this kthread from running
-	  on the specified CPUs, but (1) the kthreads may be preempted
-	  between each callback, and (2) affinity or cgroups can be used
-	  to force the kthreads to run on whatever set of CPUs is desired.
+	  and where the "p" for RCU-preempt and "s" for RCU-sched.
+	  Nothing prevents this kthread from running on the specified
+	  CPUs, but (1) the kthreads may be preempted between each
+	  callback, and (2) affinity or cgroups can be used to force
+	  the kthreads to run on whatever set of CPUs is desired.
 
 	  Say Y here if you want to help to debug reduced OS jitter.
 	  Say N here if you are unsure.

kernel/rcu/tree.c

@@ -565,7 +565,8 @@ unsigned long rcu_sched_get_gp_seq(void)
 EXPORT_SYMBOL_GPL(rcu_sched_get_gp_seq);
 
 /*
- * Return the number of RCU-bh GPs completed thus far for debug & stats.
+ * Return the number of RCU GPs completed thus far for debug & stats.
+ * This is a transitional API and will soon be removed.
  */
 unsigned long rcu_bh_get_gp_seq(void)
 {
@@ -3069,13 +3070,13 @@ void kfree_call_rcu(struct rcu_head *head,
 EXPORT_SYMBOL_GPL(kfree_call_rcu);
 
 /*
- * Because a context switch is a grace period for RCU-sched and RCU-bh,
- * any blocking grace-period wait automatically implies a grace period
- * if there is only one CPU online at any point time during execution
- * of either synchronize_sched() or synchronize_rcu_bh().  It is OK to
- * occasionally incorrectly indicate that there are multiple CPUs online
- * when there was in fact only one the whole time, as this just adds
- * some overhead: RCU still operates correctly.
+ * Because a context switch is a grace period for RCU-sched, any blocking
+ * grace-period wait automatically implies a grace period if there
+ * is only one CPU online at any point time during execution of either
+ * synchronize_sched() or synchronize_rcu_bh().  It is OK to occasionally
+ * incorrectly indicate that there are multiple CPUs online when there
+ * was in fact only one the whole time, as this just adds some overhead:
+ * RCU still operates correctly.
  */
 static int rcu_blocking_is_gp(void)
 {

kernel/rcu/update.c

@@ -298,7 +298,7 @@ EXPORT_SYMBOL_GPL(rcu_read_lock_held);
  *
  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
  *
- * Note that rcu_read_lock() is disallowed if the CPU is either idle or
+ * Note that rcu_read_lock_bh() is disallowed if the CPU is either idle or
  * offline from an RCU perspective, so check for those as well.
  */
 int rcu_read_lock_bh_held(void)