From: NeilBrown <neilb@suse.com>
Subject: SUNRPC: make lockless test safe.
Patch-mainline: never, code has changed substantually
References: bsc#1207201

When queuing a newly ready socket, we perform a lockless test of
RQ_BUSY.  That can sometimes use old data and a thread can appear to
still be busy even though it has already cleared the BUSY flag and and
checked that the queue is empty one last time.  This can result in all
threads waiting even though a socket as been queued: the ghost RQ_BUSY
prevented a wakeup.

So add a memory barrier after queuing the socket to ensure that the
subsequent test of RQ_BUSY is proplery ordered.

Signed-off-by: NeilBrown <neilb@suse.com>

---
 net/sunrpc/svc_xprt.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -460,6 +460,10 @@ redo_search:
 		list_add_tail(&xprt->xpt_ready, &pool->sp_sockets);
 		pool->sp_stats.sockets_queued++;
 		spin_unlock_bh(&pool->sp_lock);
+		/* Ensure the "lockless check" of RQ_BUSY sees rq_flags
+		 * *after* we have added to the sp_sockets queue.
+		 */
+		smp_mb();
 		goto redo_search;
 	}
 	rqstp = NULL;