[PATCH] scsi: core: don't start concurrent async scan on same host

[Ming Lei <ming.lei@redhat.com>]

Current scsi host scan mechanism supposes to fallback into sync host
scan if async scan is in-progress. However, this rule isn't strictly
respected, because scsi_prep_async_scan() doesn't hold scan_mutex when
checking shost->async_scan. When scsi_scan_host() is called
concurrently, two async scan on same host may be started, and hang in
do_scan_async() is observed.

Fixes this issue by checking & setting shost->async_scan atomically
with shost->scan_mutex.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/scsi/scsi_scan.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index f2437a7570ce..9af50e6f94c4 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -1714,15 +1714,16 @@ static void scsi_sysfs_add_devices(struct Scsi_Host *shost)
  */
 static struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
 {
-	struct async_scan_data *data;
+	struct async_scan_data *data = NULL;
 	unsigned long flags;
 
 	if (strncmp(scsi_scan_type, "sync", 4) == 0)
 		return NULL;
 
+	mutex_lock(&shost->scan_mutex);
 	if (shost->async_scan) {
 		shost_printk(KERN_DEBUG, shost, "%s called twice\n", __func__);
-		return NULL;
+		goto err;
 	}
 
 	data = kmalloc(sizeof(*data), GFP_KERNEL);
@@ -1733,7 +1734,6 @@ static struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
 		goto err;
 	init_completion(&data->prev_finished);
 
-	mutex_lock(&shost->scan_mutex);
 	spin_lock_irqsave(shost->host_lock, flags);
 	shost->async_scan = 1;
 	spin_unlock_irqrestore(shost->host_lock, flags);
@@ -1748,6 +1748,7 @@ static struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
 	return data;
 
  err:
+	mutex_unlock(&shost->scan_mutex);
 	kfree(data);
 	return NULL;
 }
-- 
2.25.2