IB/srp: Avoid that a cable pull can trigger a kernel crash

This patch fixes the following kernel crash:

general protection fault: 0000 [#1] PREEMPT SMP
Workqueue: ib_mad2 timeout_sends [ib_core]
Call Trace:
 ib_sa_path_rec_callback+0x1c4/0x1d0 [ib_core]
 send_handler+0xb2/0xd0 [ib_core]
 timeout_sends+0x14d/0x220 [ib_core]
 process_one_work+0x200/0x630
 worker_thread+0x4e/0x3b0
 kthread+0x113/0x150

Fixes: commit aef9ec39c47f ("IB: Add SCSI RDMA Protocol (SRP) initiator")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Cc: <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -665,12 +665,19 @@ static void srp_path_rec_completion(int status,
 static int srp_lookup_path(struct srp_rdma_ch *ch)
 {
 	struct srp_target_port *target = ch->target;
-	int ret;
+	int ret = -ENODEV;
 
 	ch->path.numb_path = 1;
 
 	init_completion(&ch->done);
 
+	/*
+	 * Avoid that the SCSI host can be removed by srp_remove_target()
+	 * before srp_path_rec_completion() is called.
+	 */
+	if (!scsi_host_get(target->scsi_host))
+		goto out;
+
 	ch->path_query_id = ib_sa_path_rec_get(&srp_sa_client,
 					       target->srp_host->srp_dev->dev,
 					       target->srp_host->port,
@@ -684,18 +691,24 @@ static int srp_lookup_path(struct srp_rdma_ch *ch)
 					       GFP_KERNEL,
 					       srp_path_rec_completion,
 					       ch, &ch->path_query);
-	if (ch->path_query_id < 0)
-		return ch->path_query_id;
+	ret = ch->path_query_id;
+	if (ret < 0)
+		goto put;
 
 	ret = wait_for_completion_interruptible(&ch->done);
 	if (ret < 0)
-		return ret;
+		goto put;
 
-	if (ch->status < 0)
+	ret = ch->status;
+	if (ret < 0)
 		shost_printk(KERN_WARNING, target->scsi_host,
 			     PFX "Path record query failed\n");
 
-	return ch->status;
+put:
+	scsi_host_put(target->scsi_host);
+
+out:
+	return ret;
 }
 
 static int srp_send_req(struct srp_rdma_ch *ch, bool multich)