scsi: sd: Fix a race between closing an sd device and sd I/O

The scsi_end_request() function calls scsi_cmd_to_driver() indirectly and
hence needs the disk->private_data pointer. Avoid that that pointer is
cleared before all affected I/O requests have finished. This patch avoids
that the following crash occurs:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Call trace:
 scsi_mq_uninit_cmd+0x1c/0x30
 scsi_end_request+0x7c/0x1b8
 scsi_io_completion+0x464/0x668
 scsi_finish_command+0xbc/0x160
 scsi_eh_flush_done_q+0x10c/0x170
 sas_scsi_recover_host+0x84c/0xa98 [libsas]
 scsi_error_handler+0x140/0x5b0
 kthread+0x100/0x12c
 ret_from_fork+0x10/0x18

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Jason Yan <yanaijie@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reported-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sd.c

@@ -1415,11 +1415,6 @@ static void sd_release(struct gendisk *disk, fmode_t mode)
 			scsi_set_medium_removal(sdev, SCSI_REMOVAL_ALLOW);
 	}
 
-	/*
-	 * XXX and what if there are packets in flight and this close()
-	 * XXX is followed by a "rmmod sd_mod"?
-	 */
-
 	scsi_disk_put(sdkp);
 }
 
@@ -3505,9 +3500,21 @@ static void scsi_disk_release(struct device *dev)
 {
 	struct scsi_disk *sdkp = to_scsi_disk(dev);
 	struct gendisk *disk = sdkp->disk;
-	
+	struct request_queue *q = disk->queue;
+
 	ida_free(&sd_index_ida, sdkp->index);
 
+	/*
+	 * Wait until all requests that are in progress have completed.
+	 * This is necessary to avoid that e.g. scsi_end_request() crashes
+	 * due to clearing the disk->private_data pointer. Wait from inside
+	 * scsi_disk_release() instead of from sd_release() to avoid that
+	 * freezing and unfreezing the request queue affects user space I/O
+	 * in case multiple processes open a /dev/sd... node concurrently.
+	 */
+	blk_mq_freeze_queue(q);
+	blk_mq_unfreeze_queue(q);
+
 	disk->private_data = NULL;
 	put_disk(disk);
 	put_device(&sdkp->device->sdev_gendev);