drop_monitor: Add error code to detect duplicate state changes

	Patch to add -EAGAIN error to dropwatch netlink message handling code.
-EAGAIN will be returned anytime userspace attempts to transition the state of
the drop monitor service to a state that its already in.  That allows user space
to detect this condition, so it doesn't wait for a success ACK that will never
arrive.  Tested successfully by me

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 2 deletions

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index ad41529fb60f..646ef3bc7200 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c
@@ -223,6 +223,11 @@ static int set_all_monitor_traces(int state)
 
         spin_lock(&trace_state_lock);
 
+        if (state == trace_state) {
+                rc = -EAGAIN;
+                goto out_unlock;
+        }
+
         switch (state) {
         case TRACE_ON:
                 rc |= register_trace_kfree_skb(trace_kfree_skb_hit, NULL);
@@ -251,11 +256,12 @@ static int set_all_monitor_traces(int state)
 
         if (!rc)
                 trace_state = state;
+        else
+                rc = -EINPROGRESS;
 
+out_unlock:
         spin_unlock(&trace_state_lock);
 
-        if (rc)
-                return -EINPROGRESS;
         return rc;
 }