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, 5 insertions, 2 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index f92115488187..c177ba0cce9a 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -679,7 +679,10 @@ void synchronize_rcu(void)
                            "Illegal synchronize_rcu() in RCU read-side critical section");
         if (!rcu_scheduler_active)
                 return;
-        wait_rcu_gp(call_rcu);
+        if (rcu_expedited)
+                synchronize_rcu_expedited();
+        else
+                wait_rcu_gp(call_rcu);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu);
 
@@ -831,7 +834,7 @@ void synchronize_rcu_expedited(void)
                         udelay(trycount * num_online_cpus());
                 } else {
                         put_online_cpus();
-                        synchronize_rcu();
+                        wait_rcu_gp(call_rcu);
                         return;
                 }
         }