svcrpc: fix unlikely races preventing queueing of sockets

In the rpc server, When something happens that might be reason to wake
up a thread to do something, what we do is

	- modify xpt_flags, sk_sock->flags, xpt_reserved, or
	  xpt_nr_rqsts to indicate the new situation
	- call svc_xprt_enqueue() to decide whether to wake up a thread.

svc_xprt_enqueue may require multiple conditions to be true before
queueing up a thread to handle the xprt.  In the SMP case, one of the
other CPU's may have set another required condition, and in that case,
although both CPUs run svc_xprt_enqueue(), it's possible that neither
call sees the writes done by the other CPU in time, and neither one
recognizes that all the required conditions have been set.  A socket
could therefore be ignored indefinitely.

Add memory barries to ensure that any svc_xprt_enqueue() call will
always see the conditions changed by other CPUs before deciding to
ignore a socket.

I've never seen this race reported.  In the unlikely event it happens,
another event will usually come along and the problem will fix itself.
So I don't think this is worth backporting to stable.

Chuck tried this patch and said "I don't see any performance
regressions, but my server has only a single last-level CPU cache."

Tested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
index dc1951759a8e..c35753691960 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
@@ -287,9 +287,10 @@ static void svc_rdma_wc_read_done(struct ib_cq *cq, struct ib_wc *wc)
 		spin_lock(&rdma->sc_rq_dto_lock);
 		list_add_tail(&info->ri_readctxt->rc_list,
 			      &rdma->sc_read_complete_q);
+		/* Note the unlock pairs with the smp_rmb in svc_xprt_ready: */
+		set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
 		spin_unlock(&rdma->sc_rq_dto_lock);
 
-		set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
 		svc_xprt_enqueue(&rdma->sc_xprt);
 	}