RDMA/siw: Fix serialization issue in write_space()

In siw_qp_llp_write_space(), 'sock' members should be accessed with
sk_callback_lock held, otherwise, it could race with
siw_sk_restore_upcalls(). And this could cause "NULL deref" panic.  Below
panic is due to the NULL cep returned from sk_to_cep(sk):

  Call Trace:
   <IRQ>    siw_qp_llp_write_space+0x11/0x40 [siw]
   tcp_check_space+0x4c/0xf0
   tcp_rcv_established+0x52b/0x630
   tcp_v4_do_rcv+0xf4/0x1e0
   tcp_v4_rcv+0x9b8/0xab0
   ip_protocol_deliver_rcu+0x2c/0x1c0
   ip_local_deliver_finish+0x44/0x50
   ip_local_deliver+0x6b/0xf0
   ? ip_protocol_deliver_rcu+0x1c0/0x1c0
   ip_rcv+0x52/0xd0
   ? ip_rcv_finish_core.isra.14+0x390/0x390
   __netif_receive_skb_one_core+0x83/0xa0
   netif_receive_skb_internal+0x73/0xb0
   napi_gro_frags+0x1ff/0x2b0
   t4_ethrx_handler+0x4a7/0x740 [cxgb4]
   process_responses+0x2c9/0x590 [cxgb4]
   ? t4_sge_intr_msix+0x1d/0x30 [cxgb4]
   ? handle_irq_event_percpu+0x51/0x70
   ? handle_irq_event+0x41/0x60
   ? handle_edge_irq+0x97/0x1a0
   napi_rx_handler+0x14/0xe0 [cxgb4]
   net_rx_action+0x2af/0x410
   __do_softirq+0xda/0x2a8
   do_softirq_own_stack+0x2a/0x40
   </IRQ>
   do_softirq+0x50/0x60
   __local_bh_enable_ip+0x50/0x60
   ip_finish_output2+0x18f/0x520
   ip_output+0x6e/0xf0
   ? __ip_finish_output+0x1f0/0x1f0
   __ip_queue_xmit+0x14f/0x3d0
   ? __slab_alloc+0x4b/0x58
   __tcp_transmit_skb+0x57d/0xa60
   tcp_write_xmit+0x23b/0xfd0
   __tcp_push_pending_frames+0x2e/0xf0
   tcp_sendmsg_locked+0x939/0xd50
   tcp_sendmsg+0x27/0x40
   sock_sendmsg+0x57/0x80
   siw_tx_hdt+0x894/0xb20 [siw]
   ? find_busiest_group+0x3e/0x5b0
   ? common_interrupt+0xa/0xf
   ? common_interrupt+0xa/0xf
   ? common_interrupt+0xa/0xf
   siw_qp_sq_process+0xf1/0xe60 [siw]
   ? __wake_up_common_lock+0x87/0xc0
   siw_sq_resume+0x33/0xe0 [siw]
   siw_run_sq+0xac/0x190 [siw]
   ? remove_wait_queue+0x60/0x60
   kthread+0xf8/0x130
   ? siw_sq_resume+0xe0/0xe0 [siw]
   ? kthread_bind+0x10/0x10
   ret_from_fork+0x35/0x40

Fixes: f29dd55b ("rdma/siw: queue pair methods")
Link: https://lore.kernel.org/r/20190923101112.32685-1-krishna2@chelsio.comSigned-off-by: Krishnamraju Eraparaju <krishna2@chelsio.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/siw/siw_qp.c

@@ -182,12 +182,19 @@ void siw_qp_llp_close(struct siw_qp *qp)
  */
 void siw_qp_llp_write_space(struct sock *sk)
 {
-	struct siw_cep *cep = sk_to_cep(sk);
+	struct siw_cep *cep;
 
-	cep->sk_write_space(sk);
+	read_lock(&sk->sk_callback_lock);
+
+	cep  = sk_to_cep(sk);
+	if (cep) {
+		cep->sk_write_space(sk);
 
-	if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
-		(void)siw_sq_start(cep->qp);
+		if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
+			(void)siw_sq_start(cep->qp);
+	}
+
+	read_unlock(&sk->sk_callback_lock);
 }
 
 static int siw_qp_readq_init(struct siw_qp *qp, int irq_size, int orq_size)