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, 7 insertions, 1 deletions

diff --git a/kernel/srcu.c b/kernel/srcu.c
index 97c465ebd844..de9074047c92 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -34,6 +34,10 @@
 #include <linux/delay.h>
 #include <linux/srcu.h>
 
+#include <trace/events/rcu.h>
+
+#include "rcu.h"
+
 /*
  * Initialize an rcu_batch structure to empty.
  */
@@ -464,7 +468,9 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
  */
 void synchronize_srcu(struct srcu_struct *sp)
 {
-        __synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
+        __synchronize_srcu(sp, rcu_expedited
+                           ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
+                           : SYNCHRONIZE_SRCU_TRYCOUNT);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);