scsi: core: Fix race on creating sense cache

When scsi_init_sense_cache(host) is called concurrently from different
hosts, each code path may find that no cache has been created and
allocate a new one. The lack of locking can lead to potentially
overriding a cache allocated by a different host.

Fix the issue by moving 'mutex_lock(&scsi_sense_cache_mutex)' before
scsi_select_sense_cache().

Fixes: 0a6ac4ee ("scsi: respect unchecked_isa_dma for blk-mq")
Cc: Stable <stable@vger.kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_lib.c

@@ -84,11 +84,11 @@ int scsi_init_sense_cache(struct Scsi_Host *shost)
 	struct kmem_cache *cache;
 	int ret = 0;
 
+	mutex_lock(&scsi_sense_cache_mutex);
 	cache = scsi_select_sense_cache(shost->unchecked_isa_dma);
 	if (cache)
-		return 0;
+		goto exit;
 
-	mutex_lock(&scsi_sense_cache_mutex);
 	if (shost->unchecked_isa_dma) {
 		scsi_sense_isadma_cache =
 			kmem_cache_create("scsi_sense_cache(DMA)",
@@ -104,7 +104,7 @@ int scsi_init_sense_cache(struct Scsi_Host *shost)
 		if (!scsi_sense_cache)
 			ret = -ENOMEM;
 	}
-
+ exit:
 	mutex_unlock(&scsi_sense_cache_mutex);
 	return ret;
 }