rcu: Simplify association of quiescent states with grace periods

The rdp->passed_quiesc_completed fields are used to properly
associate the recorded quiescent state with a grace period.  It
is OK to wrongly associate a given quiescent state with a
preceding grace period, but it is fatal to associate a given
quiescent state with a grace period that begins after the
quiescent state occurred.  Grace periods are numbered, and the
following fields track them:

o	->gpnum is the number of the grace period currently in
	progress, or the number of the last grace period to
	complete if no grace period is currently in progress.

o	->completed is the number of the last grace period to
	have completed.

These two fields are equal if there is no grace period in
progress, otherwise ->gpnum is one greater than ->completed.
But the rdp->passed_quiesc_completed field compared against
->completed, and if equal, the quiescent state is presumed to
count against the current grace period.

The earlier code copied rdp->completed to
rdp->passed_quiesc_completed, which has been made to work, but
is error-prone.  In contrast, copying one less than rdp->gpnum
is guaranteed safe, because rdp->gpnum is not incremented until
after the start of the corresponding grace period. At the end of
the grace period, when ->completed has incremented, then any
quiescent periods recorded previously will be discarded.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12578890421011-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/rcutree.c

@@ -100,7 +100,7 @@ void rcu_sched_qs(int cpu)
 	struct rcu_data *rdp;
 
 	rdp = &per_cpu(rcu_sched_data, cpu);
-	rdp->passed_quiesc_completed = rdp->completed;
+	rdp->passed_quiesc_completed = rdp->gpnum - 1;
 	barrier();
 	rdp->passed_quiesc = 1;
 	rcu_preempt_note_context_switch(cpu);
@@ -111,7 +111,7 @@ void rcu_bh_qs(int cpu)
 	struct rcu_data *rdp;
 
 	rdp = &per_cpu(rcu_bh_data, cpu);
-	rdp->passed_quiesc_completed = rdp->completed;
+	rdp->passed_quiesc_completed = rdp->gpnum - 1;
 	barrier();
 	rdp->passed_quiesc = 1;
 }

kernel/rcutree_plugin.h

@@ -67,7 +67,7 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
 static void rcu_preempt_qs(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
-	rdp->passed_quiesc_completed = rdp->completed;
+	rdp->passed_quiesc_completed = rdp->gpnum - 1;
 	barrier();
 	rdp->passed_quiesc = 1;
 }