scsi: core: Revert "Make sure that hosts outlive targets"

Revert the patch series "Call blk_mq_free_tag_set() earlier" because it
introduces a deadlock if the scsi_remove_host() caller holds a reference on
a device, target or host.

Link: https://lore.kernel.org/r/20220821220502.13685-4-bvanassche@acm.org
Fixes: 16728aab ("scsi: core: Make sure that hosts outlive targets")
Reported-by: syzbot+bafeb834708b1bb750bc@syzkaller.appspotmail.com
Tested-by: Kenneth R. Crudup <kenny@panix.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/hosts.c

@@ -190,13 +190,6 @@ 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);
 
@@ -406,7 +399,6 @@ 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

@@ -406,14 +406,9 @@ 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);
 }
 
@@ -528,8 +523,6 @@ 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

@@ -690,9 +690,6 @@ 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