rcu: Add a module parameter to force use of expedited RCU primitives

There have been some embedded applications that would benefit from
use of expedited grace-period primitives.  In some ways, this is
similar to synchronize_net() doing either a normal or an expedited
grace period depending on lock state, but with control outside of
the kernel.

This commit therefore adds rcu_expedited boot and sysfs parameters
that cause the kernel to substitute expedited primitives for the
normal grace-period primitives.

[ paulmck: Add trace/event/rcu.h to kernel/srcu.c to avoid build error.
	   Get rid of infinite loop through contention path.]

Signed-off-by: Antti P Miettinen <amiettinen@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 74df86bd9204..f9c17c399538 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2224,7 +2224,10 @@ void synchronize_sched(void)
 			   "Illegal synchronize_sched() in RCU-sched read-side critical section");
 	if (rcu_blocking_is_gp())
 		return;
-	wait_rcu_gp(call_rcu_sched);
+	if (rcu_expedited)
+		synchronize_sched_expedited();
+	else
+		wait_rcu_gp(call_rcu_sched);
 }
 EXPORT_SYMBOL_GPL(synchronize_sched);
 
@@ -2245,7 +2248,10 @@ void synchronize_rcu_bh(void)
 			   "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
 	if (rcu_blocking_is_gp())
 		return;
-	wait_rcu_gp(call_rcu_bh);
+	if (rcu_expedited)
+		synchronize_rcu_bh_expedited();
+	else
+		wait_rcu_gp(call_rcu_bh);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 
@@ -2328,7 +2334,7 @@ void synchronize_sched_expedited(void)
 		if (trycount++ < 10) {
 			udelay(trycount * num_online_cpus());
 		} else {
-			synchronize_sched();
+			wait_rcu_gp(call_rcu_sched);
 			return;
 		}