rcu: Determine expedited-GP IPI handler at build time

Back when there could be multiple RCU flavors running in the same kernel
at the same time, it was necessary to specify the expedited grace-period
IPI handler at runtime.  Now that there is only one RCU flavor, the
IPI handler can be determined at build time.  There is therefore no
longer any reason for the RCU-preempt and RCU-sched IPI handlers to
have different names, nor is there any reason to pass these handlers in
function arguments and in the data structures enclosing workqueues.

This commit therefore makes all these changes, pushing the specification
of the expedited grace-period IPI handler down to the point of use.

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

Documentation/RCU/Design/Expedited-Grace-Periods/ExpSchedFlow.svg

@@ -328,13 +328,13 @@
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Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html

@@ -72,10 +72,10 @@ will ignore it because idle and offline CPUs are already residing
 in quiescent states.
 Otherwise, the expedited grace period will use
 <tt>smp_call_function_single()</tt> to send the CPU an IPI, which
-is handled by <tt>sync_rcu_exp_handler()</tt>.
+is handled by <tt>rcu_exp_handler()</tt>.
 
 <p>
-However, because this is preemptible RCU, <tt>sync_rcu_exp_handler()</tt>
+However, because this is preemptible RCU, <tt>rcu_exp_handler()</tt>
 can check to see if the CPU is currently running in an RCU read-side
 critical section.
 If not, the handler can immediately report a quiescent state.
@@ -145,19 +145,18 @@ expedited grace period is shown in the following diagram:
 <p><img src="ExpSchedFlow.svg" alt="ExpSchedFlow.svg" width="55%">
 
 <p>
-As with RCU-preempt's <tt>synchronize_rcu_expedited()</tt>,
+As with RCU-preempt, RCU-sched's
 <tt>synchronize_sched_expedited()</tt> ignores offline and
 idle CPUs, again because they are in remotely detectable
 quiescent states.
-However, the <tt>synchronize_rcu_expedited()</tt> handler
-is <tt>sync_sched_exp_handler()</tt>, and because the
+However, because the
 <tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
 leave no trace of their invocation, in general it is not possible to tell
 whether or not the current CPU is in an RCU read-side critical section.
-The best that <tt>sync_sched_exp_handler()</tt> can do is to check
+The best that RCU-sched's <tt>rcu_exp_handler()</tt> can do is to check
 for idle, on the off-chance that the CPU went idle while the IPI
 was in flight.
-If the CPU is idle, then <tt>sync_sched_exp_handler()</tt> reports
+If the CPU is idle, then <tt>rcu_exp_handler()</tt> reports
 the quiescent state.
 
 <p> Otherwise, the handler forces a future context switch by setting the
@@ -298,19 +297,18 @@ Instead, the task pushing the grace period forward will include the
 idle CPUs in the mask passed to <tt>rcu_report_exp_cpu_mult()</tt>.
 
 <p>
-For RCU-sched, there is an additional check for idle in the IPI
-handler, <tt>sync_sched_exp_handler()</tt>.
+For RCU-sched, there is an additional check:
 If the IPI has interrupted the idle loop, then
-<tt>sync_sched_exp_handler()</tt> invokes <tt>rcu_report_exp_rdp()</tt>
+<tt>rcu_exp_handler()</tt> invokes <tt>rcu_report_exp_rdp()</tt>
 to report the corresponding quiescent state.
 
 <p>
 For RCU-preempt, there is no specific check for idle in the
-IPI handler (<tt>sync_rcu_exp_handler()</tt>), but because
+IPI handler (<tt>rcu_exp_handler()</tt>), but because
 RCU read-side critical sections are not permitted within the
-idle loop, if <tt>sync_rcu_exp_handler()</tt> sees that the CPU is within
+idle loop, if <tt>rcu_exp_handler()</tt> sees that the CPU is within
 RCU read-side critical section, the CPU cannot possibly be idle.
-Otherwise, <tt>sync_rcu_exp_handler()</tt> invokes
+Otherwise, <tt>rcu_exp_handler()</tt> invokes
 <tt>rcu_report_exp_rdp()</tt> to report the corresponding quiescent
 state, regardless of whether or not that quiescent state was due to
 the CPU being idle.
@@ -625,6 +623,8 @@ checks, but only during the mid-boot dead zone.
 <p>
 With this refinement, synchronous grace periods can now be used from
 task context pretty much any time during the life of the kernel.
+That is, aside from some points in the suspend, hibernate, or shutdown
+code path.
 
 <h3><a name="Summary">
 Summary</a></h3>