svcrdma: Fix race between svc_rdma_recvfrom thread and the dto_tasklet

RDMA_READ completions are kept on a separate queue from the general
I/O request queue. Since a separate lock is used to protect the RDMA_READ
completion queue, a race exists between the dto_tasklet and the
svc_rdma_recvfrom thread where the dto_tasklet sets the XPT_DATA
bit and adds I/O to the read-completion queue. Concurrently, the
recvfrom thread checks the generic queue, finds it empty and resets
the XPT_DATA bit. A subsequent svc_xprt_enqueue will fail to enqueue
the transport for I/O and cause the transport to "stall".

The fix is to protect both lists with the same lock and set the XPT_DATA
bit with this lock held.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

include/linux/sunrpc/svc_rdma.h

@@ -143,7 +143,6 @@ struct svcxprt_rdma {
 	unsigned long	     sc_flags;
 	struct list_head     sc_dto_q;		/* DTO tasklet I/O pending Q */
 	struct list_head     sc_read_complete_q;
-	spinlock_t           sc_read_complete_lock;
 	struct work_struct   sc_work;
 };
 /* sc_flags */

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -443,18 +443,18 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 
 	dprintk("svcrdma: rqstp=%p\n", rqstp);
 
-	spin_lock_bh(&rdma_xprt->sc_read_complete_lock);
+	spin_lock_bh(&rdma_xprt->sc_rq_dto_lock);
 	if (!list_empty(&rdma_xprt->sc_read_complete_q)) {
 		ctxt = list_entry(rdma_xprt->sc_read_complete_q.next,
 				  struct svc_rdma_op_ctxt,
 				  dto_q);
 		list_del_init(&ctxt->dto_q);
 	}
-	spin_unlock_bh(&rdma_xprt->sc_read_complete_lock);
-	if (ctxt)
+	if (ctxt) {
+		spin_unlock_bh(&rdma_xprt->sc_rq_dto_lock);
 		return rdma_read_complete(rqstp, ctxt);
+	}
 
-	spin_lock_bh(&rdma_xprt->sc_rq_dto_lock);
 	if (!list_empty(&rdma_xprt->sc_rq_dto_q)) {
 		ctxt = list_entry(rdma_xprt->sc_rq_dto_q.next,
 				  struct svc_rdma_op_ctxt,

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -359,11 +359,11 @@ static void sq_cq_reap(struct svcxprt_rdma *xprt)
 			if (test_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags)) {
 				struct svc_rdma_op_ctxt *read_hdr = ctxt->read_hdr;
 				BUG_ON(!read_hdr);
+				spin_lock_bh(&xprt->sc_rq_dto_lock);
 				set_bit(XPT_DATA, &xprt->sc_xprt.xpt_flags);
-				spin_lock_bh(&xprt->sc_read_complete_lock);
 				list_add_tail(&read_hdr->dto_q,
 					      &xprt->sc_read_complete_q);
-				spin_unlock_bh(&xprt->sc_read_complete_lock);
+				spin_unlock_bh(&xprt->sc_rq_dto_lock);
 				svc_xprt_enqueue(&xprt->sc_xprt);
 			}
 			svc_rdma_put_context(ctxt, 0);
@@ -428,7 +428,6 @@ static struct svcxprt_rdma *rdma_create_xprt(struct svc_serv *serv,
 	init_waitqueue_head(&cma_xprt->sc_send_wait);
 
 	spin_lock_init(&cma_xprt->sc_lock);
-	spin_lock_init(&cma_xprt->sc_read_complete_lock);
 	spin_lock_init(&cma_xprt->sc_rq_dto_lock);
 
 	cma_xprt->sc_ord = svcrdma_ord;