[PATCH RT 06/10] net: Make synchronize_rcu_expedited() conditional on !RT_FULL

[Steven Rostedt]

3.12.54-rt73-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Josh Cartwright <joshc@xxxxxx>

While the use of synchronize_rcu_expedited() might make
synchronize_net() "faster", it does so at significant cost on RT
systems, as expediting a grace period forcibly preempts any
high-priority RT tasks (via the stop_machine() mechanism).

Without this change, we can observe a latency spike up to 30us with
cyclictest by rapidly unplugging/reestablishing an ethernet link.

Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Josh Cartwright <joshc@xxxxxx>
Cc: bigeasy@xxxxxxxxxxxxx
Acked-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Acked-by: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20151027123153.GG8245@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 30fd8898bcfb..ea6092bd8219 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6231,7 +6231,7 @@ EXPORT_SYMBOL(free_netdev);
 void synchronize_net(void)
 {
 	might_sleep();
-	if (rtnl_is_locked())
+	if (rtnl_is_locked() && !IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
 		synchronize_rcu_expedited();
 	else
 		synchronize_rcu();
-- 
2.7.0