rds: Fix NULL ptr deref in xas_start

During unload of rds_rdma on older kernels, we sometimes observe the
following general protection fault (slightly edited for better
brevity):

Unregistered RDS/infiniband transport
general protection fault: 0000 [#1] SMP PTI
5.4.17-2136.330.1.oldis_combo.el8uek.v01.x86_64 #2
Workqueue: krds_cp_wq#470/0 rds_up_or_down_worker [rds]
RIP: 0010:rds_up_or_down_worker+0x54/0x2e0 [rds]

Call Trace:
 process_one_work+0x1bb/0x3a9
 worker_thread+0x37/0x3b2
 kthread+0x120/0x136
 ret_from_fork+0x2b/0x36

Using the newer uek-7-u3 kernels, v5.15.0-308.179.6.11 and
v5.15.0-311.185.2, the bug manifests itself as:

BUG: kernel NULL pointer dereference, address: 0000000000000500
Workqueue: krds_cp_wq#97/0 rds_up_or_down_worker [rds]
RIP: 0010:xas_start+0x22/0xf0
Call Trace:
 xas_load+0x8/0x91
 xa_load+0x52/0x95
 rds_ib_get_client_data+0x17/0x30 [rds_rdma]
 rds_ib_setup_qp+0x67/0xa10 [rds_rdma]
 rds_ib_cm_accept+0x105/0x360 [rds_rdma]
 rds_ib_conn_path_connect+0x1e1/0x650 [rds_rdma]
 rds_up_or_down_worker+0x1ff/0x280 [rds]
 process_one_work+0x1ee/0x3c6
 worker_thread+0x53/0x3e4
 kthread+0x127/0x144
 ? set_kthread_struct+0x60/0x52
 ret_from_fork+0x1f/0x2d

We fix this by not re-queuing the reconnect_worker, if we are in the
process of tearing the module down.

Orabug: 38169301

Fixes: ad3b8a5 ("net/rds: serialize up+down-work to relax strict ordering")
Signed-off-by: Håkon Bugge <[email protected]>
Tested-by: John Fitzgerald <[email protected]>
Tested-by: Håkon Bugge <[email protected]>
Reviewed-by: Sharath Srinivasan <[email protected]>

Author: Hakon-Bugge

net/rds/rds.h

@@ -1289,11 +1289,13 @@ static inline bool rds_cond_queue_reconnect_work(struct rds_conn_path *cp, unsig
 	unsigned long mod_delay = max(delay,
 				      msecs_to_jiffies(rds_sysctl_reconnect_max_jiffies));
 
-	if (!test_and_set_bit(RDS_RECONNECT_PENDING, &cp->cp_flags)) {
+	if (!cp->cp_conn->c_destroy_in_prog &&
+	    !test_and_set_bit(RDS_RECONNECT_PENDING, &cp->cp_flags)) {
 		rds_queue_delayed_work(cp, cp->cp_wq, &cp->cp_up_or_down_w,
 				       delay, "reconnect work");
 		return true;
-	} else if (!test_bit(RDS_SHUTDOWN_WORK_QUEUED, &cp->cp_flags) &&
+	} else if (!cp->cp_conn->c_destroy_in_prog &&
+		   !test_bit(RDS_SHUTDOWN_WORK_QUEUED, &cp->cp_flags) &&
 		   (cp->cp_up_or_down_w.timer.expires > 0) &&
 		   (cp->cp_up_or_down_w.timer.expires < KTIME_MAX) &&
 		   time_after(cp->cp_up_or_down_w.timer.expires,