scsi: core: Make sure that hosts outlive targets

Fix the race conditions between SCSI LLD kernel module unloading and SCSI
device and target removal by making sure that SCSI hosts are destroyed
after all associated target and device objects have been freed.

Link: https://lore.kernel.org/r/[email protected]
Cc: Christoph Hellwig <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Mike Christie <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: John Garry <[email protected]>
Reviewed-by: Mike Christie <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Signed-off-by: Bart Van Assche <[email protected]>
[ bvanassche: Reworked Ming's patch and split it ]
Signed-off-by: Martin K. Petersen <[email protected]>

Author: Ming Lei

drivers/scsi/hosts.c

@@ -190,6 +190,13 @@ void scsi_remove_host(struct Scsi_Host *shost)
 	transport_unregister_device(&shost->shost_gendev);
 	device_unregister(&shost->shost_dev);
 	device_del(&shost->shost_gendev);
+
+	/*
+	 * After scsi_remove_host() has returned the scsi LLD module can be
+	 * unloaded and/or the host resources can be released. Hence wait until
+	 * the dependent SCSI targets and devices are gone before returning.
+	 */
+	wait_event(shost->targets_wq, atomic_read(&shost->target_count) == 0);
 }
 EXPORT_SYMBOL(scsi_remove_host);
 
@@ -394,6 +401,7 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
 	INIT_LIST_HEAD(&shost->starved_list);
 	init_waitqueue_head(&shost->host_wait);
 	mutex_init(&shost->scan_mutex);
+	init_waitqueue_head(&shost->targets_wq);
 
 	index = ida_alloc(&host_index_ida, GFP_KERNEL);
 	if (index < 0) {

drivers/scsi/scsi_scan.c

@@ -406,9 +406,14 @@ static void scsi_target_destroy(struct scsi_target *starget)
 static void scsi_target_dev_release(struct device *dev)
 {
 	struct device *parent = dev->parent;
+	struct Scsi_Host *shost = dev_to_shost(parent);
 	struct scsi_target *starget = to_scsi_target(dev);
 
 	kfree(starget);
+
+	if (atomic_dec_return(&shost->target_count) == 0)
+		wake_up(&shost->targets_wq);
+
 	put_device(parent);
 }
 
@@ -523,6 +528,8 @@ static struct scsi_target *scsi_alloc_target(struct device *parent,
 	starget->max_target_blocked = SCSI_DEFAULT_TARGET_BLOCKED;
 	init_waitqueue_head(&starget->sdev_wq);
 
+	atomic_inc(&shost->target_count);
+
  retry:
 	spin_lock_irqsave(shost->host_lock, flags);
 

include/scsi/scsi_host.h

@@ -689,6 +689,9 @@ struct Scsi_Host {
 	/* ldm bits */
 	struct device		shost_gendev, shost_dev;
 
+	atomic_t		target_count;
+	wait_queue_head_t	targets_wq;
+
 	/*
 	 * Points to the transport data (if any) which is allocated
 	 * separately