From: Paolo Abeni <pabeni@redhat.com>
Date: Fri, 20 Apr 2018 13:18:16 +0200
Subject: tun: do not compute the rxhash, if not needed
Patch-mainline: v4.18-rc1
Git-commit: cf1a1e07fc8bb29947ad3c9568d73aee3f851431
References: bsc#1109837

Currently, the tun driver, in absence of an eBPF steering program,
always compute the rxhash in its rx path, even when such value
is later unused due to additional checks (

This changeset moves the all the related checks just before the
__skb_get_hash_symmetric(), so that the latter is no more computed
when unneeded.

Also replace an unneeded RCU section with rcu_access_pointer().

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 drivers/net/tun.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -524,11 +524,6 @@ static void tun_flow_update(struct tun_s
 
 	rcu_read_lock();
 
-	/* We may get a very small possibility of OOO during switching, not
-	 * worth to optimize.*/
-	if (tun->numqueues == 1 || tfile->detached)
-		goto unlock;
-
 	e = tun_flow_find(head, rxhash);
 	if (likely(e)) {
 		/* TODO: keep queueing to old queue until it's empty? */
@@ -547,7 +542,6 @@ static void tun_flow_update(struct tun_s
 		spin_unlock_bh(&tun->lock);
 	}
 
-unlock:
 	rcu_read_unlock();
 }
 
@@ -1937,10 +1931,13 @@ static ssize_t tun_get_user(struct tun_s
 		local_bh_enable();
 	}
 
-	rcu_read_lock();
-	if (!rcu_dereference(tun->steering_prog))
+	/* Compute the costly rx hash only if needed for flow updates.
+	 * We may get a very small possibility of OOO during switching, not
+	 * worth to optimize.
+	 */
+	if (!rcu_access_pointer(tun->steering_prog) && tun->numqueues > 1 &&
+	    !tfile->detached)
 		rxhash = __skb_get_hash_symmetric(skb);
-	rcu_read_unlock();
 
 	if (frags) {
 		/* Exercise flow dissector code path. */