[PATCH v4 2/4] scsi: core: Make sure that hosts outlive targets

[Bart Van Assche <bvanassche@acm.org>]

From: Ming Lei <ming.lei@redhat.com>

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.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: John Garry <john.garry@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
[ bvanassche: Reworked Ming's patch and split it ]
---
 drivers/scsi/hosts.c     | 8 ++++++++
 drivers/scsi/scsi_scan.c | 7 +++++++
 include/scsi/scsi_host.h | 3 +++
 3 files changed, 18 insertions(+)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index ef6c0e37acce..8fa98c8d0ee0 100644
--- a/drivers/scsi/hosts.c
+++ b/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) {
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index 4c1efd6a3b0c..ac6059702d13 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/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);
 
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 667d889b92b5..339f975d356e 100644
--- a/include/scsi/scsi_host.h
+++ b/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